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NPCIL Scientific Assistant practice questions

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Question 1Physics

In a nuclear reactor, heavy water (D2O) is used as a moderator primarily because deuterium nuclei are effective at slowing neutrons through elastic scattering while absorbing relatively few of them. Which quantity best describes a moderator's ability to reduce neutron energy per collision without excessive loss of neutrons?

  1. Thermal neutron flux divided by the fuel mass
  2. Total macroscopic absorption cross section alone
  3. Number of fissionable nuclei per unit volume
  4. Average logarithmic energy decrement divided by the absorption cross section Correct answer.

Explanation

The moderating ratio is defined as the average logarithmic energy decrement (ξ) divided by the macroscopic absorption cross section (Σa). A good moderator needs a large ξ to slow neutrons quickly and a small Σa to avoid losing them. Heavy water scores well because deuterium has a small absorption cross section despite a modest ξ.

Question 2Physics

A radioactive sample has a half-life of 20 days. Starting from an activity of 800 kBq, what is the approximate activity remaining after 60 days?

  1. 100 kBq Correct answer.
  2. 200 kBq
  3. 50 kBq
  4. 400 kBq

Explanation

Sixty days equals three half-lives (60 ÷ 20 = 3). After each half-life the activity halves, so 800 → 400 → 200 → 100 kBq. The answer is therefore 100 kBq.

Question 3Physics

In the photoelectric effect experiment, the maximum kinetic energy of emitted electrons depends on which property of the incident radiation?

  1. Area of the illuminated metal surface
  2. Frequency of the incident light Correct answer.
  3. Intensity of the incident light
  4. Angle of incidence on the metal

Explanation

Einstein's photoelectric equation gives Kmax = hν − φ, so the maximum kinetic energy depends linearly on the frequency ν of the incident light and on the work function φ of the metal. Intensity controls the number of photoelectrons, not their maximum energy. Surface area and angle affect count rate rather than the energy threshold.

Question 4Physics

A convex lens of focal length 20 cm forms a real image of an object placed 30 cm in front of it. Using the thin-lens formula, how far is the image from the lens?

  1. 50 cm
  2. 12 cm
  3. 60 cm Correct answer.
  4. 10 cm

Explanation

The thin-lens formula is 1/f = 1/v − 1/u with the sign convention giving 1/v = 1/f + 1/u. Here 1/v = 1/20 + 1/(−30) is not used directly; taking magnitudes, 1/v = 1/20 − 1/30 = 1/60, so v = 60 cm. The positive value confirms a real image on the opposite side.

Question 5Physics

An electron is accelerated through a potential difference of 100 V from rest. What is its approximate de Broglie wavelength? (Take h = 6.63×10⁻³⁴ J·s, m = 9.11×10⁻³¹ kg, e = 1.6×10⁻¹⁹ C.)

  1. 123 Å
  2. 1.23 Å Correct answer.
  3. 0.123 Å
  4. 12.3 Å

Explanation

The de Broglie wavelength for an electron accelerated through V volts is approximately λ = 12.27/√V Å. For V = 100 V, √V = 10, giving λ ≈ 1.23 Å. This matches the standard shortcut used in electron-diffraction problems.

Question 6Physics

In a nuclear fission reactor, control rods are inserted to regulate the reaction rate. Which material property makes boron, cadmium, or hafnium suitable for control rods?

  1. High thermal-neutron scattering cross section
  2. High thermal-neutron absorption cross section Correct answer.
  3. High thermal conductivity for heat removal
  4. Large average logarithmic energy decrement

Explanation

Control rods must absorb neutrons to reduce the multiplication factor and slow the chain reaction. Boron-10, cadmium-113, and hafnium isotopes have large thermal-neutron absorption cross sections. Scattering ability and moderating power are properties of moderators, not control rods.

Question 7Physics

A body moves in a circular path of radius 2 m at a constant speed of 4 m/s. What is the magnitude of its centripetal acceleration?

  1. 2 m/s²
  2. 16 m/s²
  3. 4 m/s²
  4. 8 m/s² Correct answer.

Explanation

Centripetal acceleration is a = v²/r. Substituting v = 4 m/s and r = 2 m gives a = 16/2 = 8 m/s². The direction points toward the centre of the circle.

Question 8Physics

According to the Bohr model of the hydrogen atom, the radius of the nth orbit is proportional to which power of n?

  1. 1/n
  2. 1/n²
  3. n
  4. n² Correct answer.

Explanation

In the Bohr model, the orbital radius is rn = n²a₀, where a₀ is the Bohr radius (about 0.529 Å). The radius therefore grows as the square of the principal quantum number. The energy, by contrast, varies as −1/n².

Question 9Physics

A transformer has 500 turns in the primary and 50 turns in the secondary. If the primary is connected to a 220 V AC supply, what is the secondary voltage (ideal transformer)?

  1. 220 V
  2. 22 V Correct answer.
  3. 2.2 V
  4. 2200 V

Explanation

For an ideal transformer, Vs/Vp = Ns/Np. Here Ns/Np = 50/500 = 0.1, so Vs = 0.1 × 220 V = 22 V. This is a step-down transformer.

Question 10Physics

Two point charges of +2 μC and −2 μC are separated by 0.1 m in vacuum. What is the magnitude of the electrostatic force between them? (Take 1/(4πε₀) = 9×10⁹ N·m²/C².)

  1. 0.36 N
  2. 0.036 N
  3. 36 N
  4. 3.6 N Correct answer.

Explanation

Coulomb's law gives F = k|q₁q₂|/r². Substituting q₁q₂ = 4×10⁻¹² C² and r² = 0.01 m² gives F = 9×10⁹ × 4×10⁻¹² / 0.01 = 3.6 N. The force is attractive because the charges are opposite.

Question 11Physics

A wire of resistance R is stretched uniformly to twice its original length without change in volume. What is its new resistance?

  1. R/2
  2. 4R Correct answer.
  3. 2R
  4. R

Explanation

Stretching to double length halves the cross-sectional area because volume is constant. Resistance R = ρL/A then becomes ρ(2L)/(A/2) = 4ρL/A = 4R. The resistance increases by a factor of four.

Question 12Physics

A charged particle enters a uniform magnetic field with its velocity perpendicular to the field. Which statement describes its subsequent motion?

  1. It moves in a parabola with increasing speed.
  2. It comes to rest immediately.
  3. It moves in a circle with constant speed. Correct answer.
  4. It moves in a straight line with constant speed.

Explanation

The magnetic force qv×B is perpendicular to velocity, so it does no work and the speed remains constant. A constant-magnitude force perpendicular to velocity produces uniform circular motion with radius r = mv/(qB). The particle therefore traces a circle at constant speed.

Question 13Physics

A gas is compressed isothermally at constant temperature. Which statement about the gas is correct?

  1. Its internal energy increases because work is done on it.
  2. Its internal energy decreases and heat is absorbed.
  3. Its internal energy stays constant and heat is released to the surroundings. Correct answer.
  4. No heat exchange occurs with the surroundings.

Explanation

For an ideal gas, internal energy depends only on temperature, so an isothermal process keeps ΔU = 0. By the first law, Q = ΔU + W; since work is done on the gas (W negative in the convention where W is work done by the gas), heat must be released to keep temperature constant.

Question 14Physics

A Carnot engine operates between a source at 500 K and a sink at 300 K. What is its efficiency?

  1. 60%
  2. 20%
  3. 40% Correct answer.
  4. 80%

Explanation

Carnot efficiency is η = 1 − Tc/Th = 1 − 300/500 = 0.40, or 40%. This is the maximum possible efficiency for any heat engine working between those two temperatures.

Question 15Physics

The work function of a metal is 2.0 eV. Light of frequency corresponding to photon energy 3.5 eV strikes the metal. What is the maximum kinetic energy of the emitted photoelectrons?

  1. 3.5 eV
  2. 5.5 eV
  3. 7.0 eV
  4. 1.5 eV Correct answer.

Explanation

The photoelectric equation gives Kmax = hν − φ. Substituting hν = 3.5 eV and φ = 2.0 eV yields Kmax = 1.5 eV. Emission occurs because the photon energy exceeds the work function.

Question 16Physics

In a nuclear reactor, the multiplication factor k is defined as the ratio of neutrons in one generation to those in the preceding generation. Which condition corresponds to a self-sustaining critical reactor?

  1. k = 1 Correct answer.
  2. k > 1
  3. k = 0
  4. k < 1

Explanation

A reactor is critical when each fission generation produces exactly as many neutrons as the previous one, i.e. k = 1. Subcritical operation (k < 1) leads to a decaying chain reaction, while supercritical operation (k > 1) causes the neutron population to grow.

Question 17Physics

A radioactive nucleus undergoes alpha decay. How do its mass number A and atomic number Z change?

  1. A is unchanged and Z decreases by 2
  2. A decreases by 4 and Z is unchanged
  3. A decreases by 2 and Z decreases by 4
  4. A decreases by 4 and Z decreases by 2 Correct answer.

Explanation

An alpha particle is a helium nucleus with A = 4 and Z = 2. Emitting it reduces the parent's mass number by 4 and atomic number by 2, following conservation of nucleons and charge.

Question 18Physics

A simple pendulum has a period of 2 s on Earth. If it is taken to a location where the acceleration due to gravity is one-fourth of that on Earth, what is its new period?

  1. 8 s
  2. 2 s
  3. 4 s Correct answer.
  4. 1 s

Explanation

The period of a simple pendulum is T = 2π√(L/g), so T is inversely proportional to √g. If g becomes g/4, then √g becomes √g/2 and the period doubles. Thus the new period is 4 s.

Question 19Physics

A sound wave travels from air into water. Which quantity remains unchanged across the boundary?

  1. Frequency Correct answer.
  2. Wavelength
  3. Amplitude
  4. Speed

Explanation

Frequency is set by the source and stays constant when a wave crosses into a new medium. Speed and wavelength change because the wave speed is different in water, with wavelength adjusting so that f remains fixed. Amplitude may change due to reflection at the interface.

Question 20Physics

In a Young's double-slit experiment, the slit separation is doubled while the screen distance and wavelength remain the same. What happens to the fringe width?

  1. It is halved. Correct answer.
  2. It becomes four times larger.
  3. It doubles.
  4. It remains the same.

Explanation

Fringe width is given by β = λD/d. Doubling the slit separation d while keeping λ and D constant halves the fringe width. The fringes therefore move closer together.

Question 21Physics

A particle of mass 2 kg moving at 3 m/s collides head-on and sticks to a stationary particle of mass 4 kg. What is the velocity of the combined mass after the perfectly inelastic collision?

  1. 1 m/s Correct answer.
  2. 3 m/s
  3. 1.5 m/s
  4. 2 m/s

Explanation

Momentum is conserved in a perfectly inelastic collision: m₁u₁ = (m₁ + m₂)v. Substituting 2 × 3 = 6 kg·m/s and dividing by the total mass 6 kg gives v = 1 m/s. Kinetic energy is not conserved here.

Question 22Physics

The binding energy per nucleon curve peaks near iron-56 (about 8.8 MeV per nucleon). What does this imply about energy release in nuclear reactions?

  1. Only fission of heavy nuclei releases energy.
  2. Neither fission nor fusion releases energy near iron.
  3. Fission of heavy nuclei and fusion of light nuclei both release energy. Correct answer.
  4. Only fusion of light nuclei releases energy.

Explanation

Nuclei lighter than iron release energy when they fuse toward the peak, and nuclei heavier than iron release energy when they split toward the peak. Both processes increase the binding energy per nucleon of the products. Reactions that move nucleons toward iron-56 therefore liberate energy.

Question 23Physics

A parallel-plate capacitor has capacitance C. If the plate separation is halved and the plate area is doubled, what is the new capacitance?

  1. C/2
  2. 4C Correct answer.
  3. 2C
  4. C/4

Explanation

Capacitance is C = ε₀A/d. Doubling the area doubles C, and halving the separation doubles it again. The combined effect is a factor of four, giving 4C.

Question 24Physics

In a series LCR circuit at resonance, which statement is correct?

  1. The current leads the voltage by 90°.
  2. The inductive and capacitive reactances add to give maximum impedance.
  3. The impedance equals the resistance and current is maximum. Correct answer.
  4. The impedance is zero and current is infinite.

Explanation

At resonance the inductive and capacitive reactances are equal and opposite, so they cancel. The impedance then reduces to the resistance R, which is its minimum value, and the current reaches its maximum. The circuit is purely resistive at resonance.

Question 25Physics

A nucleus of uranium-235 absorbs a thermal neutron and undergoes fission, releasing on average about 200 MeV per fission. Approximately how many fissions per second are needed to produce a thermal power of 100 MW? (Take 1 MeV = 1.6×10⁻¹³ J.)

  1. About 3×10¹⁵ per second
  2. About 3×10¹⁸ per second Correct answer.
  3. About 3×10¹² per second
  4. About 3×10²¹ per second

Explanation

Each fission releases about 200 × 1.6×10⁻¹³ J = 3.2×10⁻¹¹ J. For 100 MW = 1×10⁸ W, the fission rate is 1×10⁸ / 3.2×10⁻¹¹ ≈ 3×10¹⁸ fissions per second. This illustrates the enormous energy density of nuclear fuel.

Question 26Physics

A Geiger-Müller counter registers a background count of 20 counts per minute. A source is brought near and the counter records 140 counts per minute. What is the corrected count rate due to the source alone?

  1. 120 counts per minute Correct answer.
  2. 100 counts per minute
  3. 140 counts per minute
  4. 160 counts per minute

Explanation

Background radiation is always present and must be subtracted from the measured rate to isolate the source contribution. The observed rate is 140 cpm and the background is 20 cpm, so the corrected source rate is 140 − 20 = 120 cpm.

Question 27Physics

Which quantity is conserved in every nuclear reaction, in addition to electric charge and linear momentum?

  1. Total number of electrons
  2. Total kinetic energy
  3. Total number of nucleons Correct answer.
  4. Total number of protons only

Explanation

Nuclear reactions conserve the total nucleon number (mass number A) as well as charge and momentum. Kinetic energy is not generally conserved because it is converted to or from mass energy, and proton number alone changes in beta decay.

Question 28Physics

A nuclide has a half-life of 8 days. What fraction of an initially pure sample remains undecayed after 24 days?

  1. 1/8 Correct answer.
  2. 1/16
  3. 1/3
  4. 1/4

Explanation

The elapsed time of 24 days equals three half-lives (24 ÷ 8 = 3). Each half-life halves the remaining amount, so the fraction left is (1/2)^3 = 1/8.

Question 29Physics

In a nuclear power reactor, heavy water (D2O) is used as a moderator. What is its primary function?

  1. To slow fast fission neutrons to thermal energies without absorbing many of them Correct answer.
  2. To shield personnel from gamma radiation
  3. To absorb surplus neutrons and shut down the chain reaction
  4. To transfer heat from the core to the steam generator

Explanation

A moderator reduces the kinetic energy of fast neutrons through elastic scattering so that they are more likely to induce further fission in uranium-235. Heavy water is effective because its low neutron absorption keeps the multiplication factor high.

Question 30Physics

A radioactive source gives a count rate of 4800 counts per minute at a certain distance from a detector. At twice that distance, what count rate is expected, ignoring background?

  1. 2400 counts per minute
  2. 600 counts per minute
  3. 9600 counts per minute
  4. 1200 counts per minute Correct answer.

Explanation

For a point source the intensity follows the inverse-square law, so doubling the distance reduces the count rate by a factor of four. Thus 4800 ÷ 4 = 1200 counts per minute.

Question 31Physics

In a nuclear fission event, a uranium-235 nucleus releases about 200 MeV of energy. Approximately how much mass is converted into energy per fission? (1 u = 931.5 MeV)

  1. About 2.1 u
  2. About 0.21 u Correct answer.
  3. About 0.021 u
  4. About 21 u

Explanation

Using E = mc² in atomic mass units, the mass defect is 200 ÷ 931.5 ≈ 0.215 u. This small fraction of the uranium nucleus mass accounts for the large energy release.

Question 32Physics

A beam of electrons is accelerated through a potential difference and then diffracted by a crystal lattice, producing a pattern of rings. This observation most directly demonstrates which property of the electrons?

  1. Quantisation of electric charge
  2. Existence of a discrete energy level structure in the crystal
  3. Wave nature of matter Correct answer.
  4. Relativistic increase of electron mass

Explanation

Diffraction is a wave phenomenon, so electron diffraction confirms the de Broglie wave nature of moving particles. The de Broglie wavelength λ = h/p determines the observed diffraction geometry.

Question 33Physics

A photon of energy 5.0 eV is absorbed by a hydrogen atom in its ground state (ionisation energy 13.6 eV). What happens to the atom?

  1. It is excited to a higher bound level and later emits a photon Correct answer.
  2. It undergoes nuclear excitation and emits a gamma ray
  3. It is ionised and the electron leaves with kinetic energy
  4. It remains in the ground state and the photon passes through

Explanation

The ground-state hydrogen atom has levels at −13.6 eV, −3.4 eV, −1.51 eV, and so on, so an absorbed 5.0 eV photon can lift the electron to a higher bound level. The atom is excited, not ionised, because 5.0 eV is less than the 13.6 eV ionisation energy.

Question 34Physics

In a nuclear reactor, delayed neutrons are important because they

  1. make the reactor controllable by lengthening the effective neutron generation time Correct answer.
  2. carry most of the fission energy released in the core
  3. are the only neutrons capable of causing fast fission in uranium-238
  4. prevent the formation of plutonium-239 from uranium-238

Explanation

A small fraction of fission neutrons is emitted seconds after fission by decaying fission products. This delays part of the neutron population and increases the effective generation time, giving operators time to respond with control rods.

Question 35Physics

A charged particle moves in a uniform magnetic field with its velocity perpendicular to the field. The radius of its circular path is r. If the particle's speed is doubled while the same field is maintained, what is the new radius?

  1. 2r Correct answer.
  2. 4r
  3. r/2
  4. r

Explanation

The magnetic force provides the centripetal force, giving qvB = mv²/r, so r = mv/(qB). The radius is directly proportional to speed, so doubling the speed doubles the radius.

Question 36Physics

A sample contains two radionuclides: one with a half-life of 2 hours and another with a half-life of 20 hours, initially present in equal activities. After 20 hours, which statement about their activities is correct?

  1. The short-lived nuclide now dominates the activity
  2. Both nuclides have decayed completely
  3. Both nuclides have the same activity as initially
  4. The short-lived nuclide contributes almost nothing, and the long-lived one has fallen to about half its initial activity Correct answer.

Explanation

After 20 hours the 2-hour nuclide has passed through ten half-lives, so its activity is reduced by a factor of about 1024. The 20-hour nuclide has passed through one half-life, so its activity is about half the initial value, and it dominates the total.

Question 37Physics

In a nuclear reactor, a control rod made of boron is inserted further into the core. What is the immediate effect on the neutron multiplication factor k?

  1. k decreases because more thermal neutrons are absorbed Correct answer.
  2. k decreases because boron slows down the neutrons
  3. k increases because more fast neutrons are produced
  4. k remains unchanged because boron affects only gamma rays

Explanation

Boron-10 has a large thermal neutron capture cross-section, so inserting it removes neutrons from the chain reaction and lowers k. Slowing neutrons is the moderator's job, while absorption reduces the neutron population directly.

Question 38Physics

A body of mass 2 kg is moving with a velocity of (3i + 4j) m/s. What is the magnitude of its momentum?

  1. 5 kg m/s
  2. 10 kg m/s Correct answer.
  3. 7 kg m/s
  4. 14 kg m/s

Explanation

The magnitude of the velocity is √(3² + 4²) = 5 m/s. Momentum magnitude is mv = 2 × 5 = 10 kg m/s.

Question 39Physics

A thin convex lens of focal length 10 cm forms an image of an object placed 15 cm from the lens. Using the lens formula, what is the image distance and nature of the image?

  1. 30 cm on the same side, virtual and erect
  2. 15 cm on the opposite side, real and inverted
  3. 30 cm on the opposite side, real and inverted Correct answer.
  4. 6 cm on the opposite side, real and inverted

Explanation

The lens formula gives 1/v = 1/f − 1/u = 1/10 − 1/15 = 1/30, so v = 30 cm. A positive image distance for a convex lens means a real, inverted image on the opposite side of the lens.

Question 40Physics

A series LCR circuit has R = 10 Ω, L = 0.1 H, and C = 100 μF. At the resonant frequency, what is the impedance and the phase relationship between current and supply voltage?

  1. Zero, with current leading voltage by 90°
  2. 10 Ω, with current lagging voltage by 90°
  3. 10 Ω, with current in phase with voltage Correct answer.
  4. Infinite, with current lagging voltage by 90°

Explanation

At resonance the inductive and capacitive reactances are equal and cancel, so the impedance equals the resistance R = 10 Ω. The circuit is purely resistive at resonance, so the current is in phase with the applied voltage.

Question 41Chemistry

In a nuclear power plant, heavy water (D2O) is used as a moderator because deuterium nuclei are particularly effective at slowing neutrons. Which property of deuterium best explains this efficiency?

  1. Its mass is close to that of a neutron, so collisions transfer energy efficiently, and it absorbs very few neutrons Correct answer.
  2. It has a large neutron absorption cross-section, which removes fast neutrons from the core
  3. It undergoes fission readily, releasing additional neutrons that sustain the chain reaction
  4. It has a high electrical conductivity, which allows neutrons to be slowed by electromagnetic forces

Explanation

Moderation works by elastic scattering, and a moderator nucleus whose mass is comparable to the neutron's mass removes the most energy per collision. Deuterium has almost the same mass as a neutron and also has a very small absorption cross-section, so it slows neutrons without consuming them. Heavy water therefore gives good moderation with low neutron loss.

Question 42Chemistry

The pH of a 0.01 M aqueous solution of a strong monobasic acid at 25 degrees Celsius is:

  1. 7
  2. 1
  3. 12
  4. 2 Correct answer.

Explanation

A strong monobasic acid is fully dissociated, so a 0.01 M solution gives a hydrogen ion concentration of 1 x 10^-2 mol per litre. pH is the negative logarithm of that concentration, giving pH = 2. The other values correspond to different concentrations or to a basic solution.

Question 43Chemistry

Which of the following is the correct electron configuration of a neutral chlorine atom (Z = 17) in its ground state?

  1. 1s2 2s2 2p6 3s1 3p6
  2. 1s2 2s2 2p5 3s2 3p6
  3. 1s2 2s2 2p6 3s2 3p6
  4. 1s2 2s2 2p6 3s2 3p5 Correct answer.

Explanation

Chlorine has 17 electrons, which fill orbitals in order of increasing energy: 1s, 2s, 2p, 3s and then 3p. This gives 2 + 2 + 6 + 2 + 5 = 17 electrons, ending in 3p5. One vacancy in the 3p subshell accounts for chlorine's typical valency of one.

Question 44Chemistry

In the periodic table, atomic radius generally decreases across a period from left to right. Which factor is mainly responsible for this trend?

  1. Decreasing nuclear charge combined with addition of new electron shells
  2. Decreasing number of protons in the nucleus across the period
  3. Increasing nuclear charge with electrons added to the same principal shell Correct answer.
  4. Increasing shielding of valence electrons by inner-shell electrons

Explanation

Across a period, protons are added to the nucleus while new electrons enter the same principal shell, so the effective nuclear charge felt by valence electrons rises. The increased attraction pulls the electron cloud closer, reducing atomic radius. Shielding changes only slightly because the added electrons are in the same shell.

Question 45Chemistry

Which type of chemical bond is present in solid sodium chloride?

  1. Ionic bonding formed by transfer of an electron from sodium to chlorine Correct answer.
  2. Covalent bonding between sodium and chlorine atoms sharing electron pairs
  3. Hydrogen bonding between sodium ions and chloride ions
  4. Metallic bonding between a lattice of sodium cations and delocalised electrons

Explanation

Sodium has low ionisation energy and chlorine has high electron affinity, so an electron is transferred from sodium to chlorine. The resulting Na+ and Cl- ions are held in a lattice by strong electrostatic attraction, which is ionic bonding. This accounts for the high melting point and electrical conduction when molten.

Question 46Chemistry

Which of the following statements correctly describes an exothermic reaction?

  1. It always proceeds spontaneously regardless of temperature
  2. It absorbs heat from the surroundings, so the surroundings become cooler
  3. It produces no change in the enthalpy of the system
  4. It releases heat to the surroundings, so the enthalpy change is negative Correct answer.

Explanation

In an exothermic reaction the system loses enthalpy to the surroundings, so the surroundings warm up and the enthalpy change of the reaction is negative. The sign convention follows from defining enthalpy lost by the system as a negative change. Spontaneity depends on both enthalpy and entropy, not on the sign of the enthalpy change alone.

Question 47Chemistry

In the reaction N2(g) + 3H2(g) <-> 2NH3(g), which change in conditions shifts the equilibrium towards the formation of ammonia?

  1. Increasing the temperature of the system
  2. Adding an inert gas at constant volume
  3. Increasing the total pressure of the system Correct answer.
  4. Decreasing the total pressure of the system

Explanation

The forward reaction converts four moles of gas into two moles, so it reduces the number of gas molecules. Raising the pressure favours the direction that produces fewer gas molecules, which is ammonia formation. Adding inert gas at constant volume does not change partial pressures of the reactants.

Question 48Chemistry

Which of the following is the correct IUPAC name for the compound CH3-CH2-CH2-OH?

  1. Propan-2-ol
  2. Propanoic acid
  3. Propanal
  4. Propan-1-ol Correct answer.

Explanation

The chain contains three carbon atoms, giving the stem propane, and the hydroxyl group is on the terminal carbon, so the suffix is -1-ol. The compound is therefore propan-1-ol. The hydroxyl group defines an alcohol rather than an aldehyde or carboxylic acid.

Question 49Chemistry

A sample of an ideal gas occupies 2.0 litres at 300 K and 1.0 bar. If the temperature is raised to 600 K at constant pressure, the new volume will be:

  1. 8.0 litres
  2. 1.0 litre
  3. 2.0 litres
  4. 4.0 litres Correct answer.

Explanation

Charles's law states that at constant pressure the volume of a fixed amount of gas is directly proportional to absolute temperature. Doubling the temperature from 300 K to 600 K therefore doubles the volume from 2.0 litres to 4.0 litres. Temperatures must be in kelvin for this proportionality to hold.

Question 50Chemistry

In the titration of 25.0 mL of 0.10 M HCl with 0.10 M NaOH, what volume of NaOH solution is required to reach the equivalence point?

  1. 12.5 mL
  2. 50.0 mL
  3. 25.0 mL Correct answer.
  4. 2.5 mL

Explanation

At the equivalence point the moles of acid equal the moles of base. Since both solutions have the same concentration and the reaction is one-to-one, equal volumes are needed, so 25.0 mL of NaOH is required. The moles of HCl present are 0.0025 mol, which requires the same number of moles of NaOH.