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NEB Class 12 Chemistry study materials
NEB Class 12 Chemistry study resources

NEB Class 12 Chemistry practice questions

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Question 1General and Physical Chemistry

How many significant figures are present in the measured value 0.004560 g?

  1. 3
  2. 6
  3. 4 Correct answer.
  4. 7

Explanation

Leading zeros that only fix the decimal point are never significant, so the figures begin at the 4. The digits 4, 5, 6 and the trailing zero after the decimal are all significant, giving four significant figures.

Question 2General and Physical Chemistry

Which statement correctly describes the value of the Avogadro constant?

  1. The mass in grams of one molecule of hydrogen
  2. The number of particles in one mole of any substance Correct answer.
  3. The number of molecules in 22.4 L of any gas at any temperature
  4. The number of atoms in exactly 1 g of carbon-12

Explanation

One mole is defined as the amount of substance containing as many elementary entities as there are atoms in 12 g of carbon-12, and that number is the Avogadro constant, about 6.022 × 10^23 per mole. It applies to any species, not just carbon or gases at one particular temperature.

Question 3General and Physical Chemistry

A sample of a compound contains 40% carbon, 6.7% hydrogen and 53.3% oxygen by mass. What is its empirical formula? (C = 12, H = 1, O = 16)

  1. C2H4O
  2. CHO
  3. CH2O Correct answer.
  4. C2H6O

Explanation

Dividing each percentage by the atomic mass gives mole ratios of 3.33 : 6.7 : 3.33 for C : H : O. Dividing by the smallest value gives 1 : 2 : 1, so the empirical formula is CH2O.

Question 4General and Physical Chemistry

In the reaction N2(g) + 3H2(g) → 2NH3(g), 28 g of nitrogen reacts with 6 g of hydrogen. Which reactant is limiting and what mass of ammonia forms? (N = 14, H = 1)

  1. N2 is limiting; 17 g of NH3 forms
  2. H2 is limiting; 17 g of NH3 forms
  3. H2 is limiting; 34 g of NH3 forms Correct answer.
  4. N2 is limiting; 34 g of NH3 forms

Explanation

28 g of N2 is 1 mol and 6 g of H2 is 3 mol, which is exactly the 1 : 3 stoichiometric ratio, so neither is in excess under these exact amounts; however, 1 mol N2 requires 3 mol H2 and gives 2 mol NH3, i.e. 34 g. Since the hydrogen amount is fully consumed in producing that ammonia, hydrogen is the reagent whose quantity fixes the yield here.

Question 5General and Physical Chemistry

Which set of quantum numbers is not permitted for an electron in an atom?

  1. n = 3, l = 2, m = 0
  2. n = 2, l = 1, m = -1
  3. n = 4, l = 0, m = 0
  4. n = 3, l = 3, m = 1 Correct answer.

Explanation

The azimuthal quantum number l can only take values from 0 up to n − 1. For n = 3 the allowed values are 0, 1 and 2, so l = 3 is impossible.

Question 6General and Physical Chemistry

Which statement about the shapes of molecules predicted by VSEPR theory is correct?

  1. H2O is linear because oxygen forms two bonds
  2. NH3 is trigonal planar because nitrogen forms three bonds
  3. BF3 is pyramidal with bond angles of 107°
  4. CH4 is tetrahedral with bond angles of 109.5° Correct answer.

Explanation

In CH4 the carbon has four bonding pairs and no lone pairs, so the electron pairs arrange tetrahedrally with bond angles of about 109.5°. Lone pairs, where present, distort the ideal geometry.

Question 7General and Physical Chemistry

Which of the following species has the electron configuration 1s² 2s² 2p⁶?

  1. Ne⁻
  2. F⁺
  3. O²⁺
  4. Na⁺ Correct answer.

Explanation

A sodium atom has 11 electrons, and removing one electron to form Na⁺ leaves 10 electrons, matching the configuration of neon. The other species do not have exactly ten electrons.

Question 8General and Physical Chemistry

Which periodic trend is correctly described across a period from left to right?

  1. Atomic radius increases because nuclear charge increases
  2. Electronegativity decreases because atomic size increases
  3. Metallic character increases because ionisation enthalpy falls
  4. Ionisation enthalpy generally increases because effective nuclear charge increases Correct answer.

Explanation

Across a period the nuclear charge rises while electrons enter the same shell, so the effective nuclear charge felt by outer electrons increases and removing an electron becomes harder. Hence ionisation enthalpy generally increases.

Question 9General and Physical Chemistry

Which molecule contains both sigma and pi bonds, with one sigma and two pi bonds between the two carbon atoms?

  1. Hydrogen, H2
  2. Ethyne, C2H2 Correct answer.
  3. Ethene, C2H4
  4. Ethane, C2H6

Explanation

A triple bond consists of one sigma bond formed by head-on overlap and two pi bonds formed by sideways overlap of unhybridised p orbitals. Ethyne has a carbon–carbon triple bond, so it fits this description.

Question 10General and Physical Chemistry

Which pair of conditions shifts the equilibrium of the exothermic reaction N2(g) + 3H2(g) ⇌ 2NH3(g) towards more ammonia?

  1. Increasing temperature and decreasing pressure
  2. Decreasing temperature and decreasing pressure
  3. Increasing temperature and increasing pressure
  4. Decreasing temperature and increasing pressure Correct answer.

Explanation

By Le Chatelier's principle, lowering the temperature favours the exothermic forward reaction, and raising the pressure favours the side with fewer gas moles, which is the product side (2 moles versus 4). Combining both changes increases the ammonia yield.

Question 11General and Physical Chemistry

For the equilibrium reaction H2(g) + I2(g) ⇌ 2HI(g), what is the effect of increasing the total pressure at constant temperature?

  1. The equilibrium shifts towards HI
  2. The value of Kp increases
  3. The equilibrium position is unchanged Correct answer.
  4. The equilibrium shifts towards H2 and I2

Explanation

There are two moles of gas on each side of the equation, so Δn = 0 and the pressure term in the equilibrium expression cancels. Changing the total pressure therefore does not shift the position of this equilibrium.

Question 12General and Physical Chemistry

Which statement about a catalyst is correct?

  1. It lowers the activation energy by providing an alternative reaction pathway Correct answer.
  2. It is consumed permanently during the reaction
  3. It increases the enthalpy change of the reaction
  4. It increases the equilibrium constant of a reversible reaction

Explanation

A catalyst provides an alternative pathway of lower activation energy, so a larger fraction of collisions is effective and the rate increases. It does not change the thermodynamics of the reaction.

Question 13General and Physical Chemistry

For a first-order reaction with rate constant k = 0.0693 min⁻¹, what is the half-life?

  1. 5 min
  2. 10 min Correct answer.
  3. 0.1 min
  4. 20 min

Explanation

For a first-order reaction the half-life is t½ = 0.693/k, independent of initial concentration. Substituting k = 0.0693 min⁻¹ gives t½ = 0.693/0.0693 = 10 min.

Question 14General and Physical Chemistry

Which factor does not affect the rate of a chemical reaction at constant temperature?

  1. Concentration of reactants
  2. Presence of a catalyst
  3. Surface area of a solid reactant
  4. Total volume of the container for a reaction in solution Correct answer.

Explanation

For reactions in solution, changing the total volume while keeping the amounts and concentrations the same does not change the collision frequency. Concentration, surface area and catalysts all directly affect the rate.

Question 15General and Physical Chemistry

Which statement about the first law of thermodynamics is correct?

  1. The enthalpy change is always zero for a spontaneous process
  2. Heat always flows from a colder to a hotter body
  3. Energy can be created in an exothermic reaction
  4. The internal energy change equals heat absorbed plus work done on the system Correct answer.

Explanation

The first law states that the change in internal energy equals the heat added to the system plus the work done on it, ΔU = q + w. Energy is conserved and is neither created nor destroyed.

Question 16General and Physical Chemistry

For a reaction to be spontaneous at all temperatures, which combination of signs of ΔH and ΔS is required?

  1. ΔH positive, ΔS positive
  2. ΔH positive, ΔS negative
  3. ΔH negative, ΔS positive Correct answer.
  4. ΔH negative, ΔS negative

Explanation

Spontaneity requires ΔG = ΔH − TΔS to be negative. With ΔH negative and ΔS positive, both terms make ΔG negative at every temperature, so the reaction is always spontaneous.

Question 17General and Physical Chemistry

Which of the following is a state function?

  1. Heat
  2. Work
  3. Path length of a reaction
  4. Enthalpy Correct answer.

Explanation

A state function depends only on the current state of the system and not on how that state was reached. Enthalpy is a state function, whereas heat and work depend on the path taken.

Question 18General and Physical Chemistry

Which statement about electrochemical cells is correct?

  1. In a galvanic cell, electrons flow from the anode to the cathode through the external circuit Correct answer.
  2. The salt bridge carries electrons between the half-cells
  3. Oxidation occurs at the cathode in a galvanic cell
  4. Reduction always occurs at the anode in an electrolytic cell

Explanation

By definition, the anode is where oxidation occurs and releases electrons, so in a galvanic cell electrons travel through the external wire from anode to cathode. The salt bridge carries ions, not electrons.

Question 19General and Physical Chemistry

The standard electrode potential of Zn²⁺/Zn is −0.76 V and that of Cu²⁺/Cu is +0.34 V. What is the standard cell potential of the Daniell cell?

  1. 1.10 V Correct answer.
  2. −1.10 V
  3. −0.42 V
  4. 0.42 V

Explanation

The cell potential is E°cathode − E°anode, with copper acting as cathode because it has the more positive potential. Thus E°cell = 0.34 − (−0.76) = 1.10 V, and the positive value confirms a spontaneous reaction.

Question 20General and Physical Chemistry

What is the pH of a 0.01 M solution of a strong monobasic acid?

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

Explanation

A strong monobasic acid dissociates completely, so [H⁺] equals the acid concentration, 0.01 M or 10⁻² M. Since pH = −log[H⁺], the pH is 2.

Question 21General and Physical Chemistry

Which statement about buffer solutions is correct?

  1. They have a pH that is always exactly 7
  2. They neutralise any amount of added acid without change
  3. They consist of a strong acid and its salt
  4. They resist changes in pH when small amounts of acid or base are added Correct answer.

Explanation

A buffer contains a weak acid and its conjugate base, or a weak base and its conjugate acid, which can absorb added H⁺ or OH⁻ with only a small pH change. Its pH depends on the ratio of the components, not on neutrality.

Question 22General and Physical Chemistry

Which of the following is the conjugate base of the hydrogen carbonate ion, HCO3⁻?

  1. OH⁻
  2. H2CO3
  3. CO3²⁻ Correct answer.
  4. CO2

Explanation

A conjugate base is formed by removing one proton from the acid. Removing H⁺ from HCO3⁻ leaves CO3²⁻, so carbonate is the conjugate base.

Question 23General and Physical Chemistry

Which statement about the solubility product of a sparingly soluble salt MX with formula MX2 is correct?

  1. Ksp = [M²⁺][X⁻]³
  2. Ksp = [M²⁺][X⁻]
  3. Ksp = [M²⁺]²[X⁻]
  4. Ksp = [M²⁺][X⁻]² Correct answer.

Explanation

For the dissolution MX2(s) ⇌ M²⁺(aq) + 2X⁻(aq), each ion concentration is raised to the power of its stoichiometric coefficient. Hence Ksp = [M²⁺][X⁻]².

Question 24General and Physical Chemistry

Which statement about the properties of colloids is correct?

  1. Tyndall scattering of light is a characteristic property of colloidal solutions Correct answer.
  2. Colloidal dispersions are always transparent and show no scattering
  3. Colloidal particles settle out on standing without any treatment
  4. Colloidal particles pass through semipermeable membranes

Explanation

Colloidal particles are large enough to scatter light, producing the Tyndall effect, which is used to distinguish a colloid from a true solution. They are small enough, however, to remain suspended by Brownian motion.

Question 25General and Physical Chemistry

For the reaction A + B → products, doubling [A] doubles the rate while doubling [B] quadruples the rate. What is the overall order of the reaction?

  1. Third order Correct answer.
  2. First order
  3. Zero order
  4. Second order

Explanation

Doubling [A] doubles the rate, so the reaction is first order in A. Doubling [B] quadruples the rate, so it is second order in B, giving an overall order of 1 + 2 = 3.

Question 26General and Physical Chemistry

How many significant figures are present in the measured value 0.004560 m?

  1. 4 Correct answer.
  2. 6
  3. 3
  4. 7

Explanation

Leading zeros that only locate the decimal point are not significant, but the trailing zero after the 6 is significant because it follows a decimal value. The significant digits are 4, 5, 6 and the final 0, giving four.

Question 27General and Physical Chemistry

An element has the ground-state electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹. To which block and group of the periodic table does it belong?

  1. p-block, group 1
  2. s-block, group 2
  3. s-block, group 1 Correct answer.
  4. d-block, group 1

Explanation

The last electron enters the 4s orbital, so the element is in the s-block. One electron in the outermost s orbital places it in group 1, and the configuration corresponds to potassium.

Question 28General and Physical Chemistry

Which quantity is conserved when an ideal gas expands isothermally and reversibly?

  1. Temperature only
  2. Internal energy only
  3. Enthalpy only
  4. Temperature and internal energy Correct answer.

Explanation

For an ideal gas, internal energy depends only on temperature, so isothermal expansion keeps both temperature and internal energy constant. Enthalpy also stays constant, but the option stating temperature and internal energy is the complete correct pairing among the choices given.

Question 29General and Physical Chemistry

Which expression gives the pH of a 0.001 M aqueous solution of NaOH at 25 °C, assuming complete dissociation?

  1. pH = 14
  2. pH = 3
  3. pH = 7
  4. pH = 11 Correct answer.

Explanation

Complete dissociation gives [OH⁻] = 10⁻³ M, so pOH = 3 and pH = 14 − 3 = 11. The solution is basic, consistent with a pH above 7.

Question 30General and Physical Chemistry

Which statement correctly describes the rate of a chemical reaction?

  1. It is the change in concentration of a reactant per unit time. Correct answer.
  2. It is the equilibrium constant of the reaction.
  3. It is the time taken for all reactants to be used up.
  4. It is the total amount of reactant consumed at the end of the reaction.

Explanation

Reaction rate measures how fast concentration changes with time, usually expressed as decrease in reactant or increase in product concentration per unit time. It is a rate, not a total quantity or an equilibrium constant.

Question 31General and Physical Chemistry

Which pair of species is isoelectronic with the neon atom?

  1. Na⁺ and Cl⁻
  2. K⁺ and F⁻
  3. Mg²⁺ and Cl⁻
  4. Na⁺ and F⁻ Correct answer.

Explanation

Neon has 10 electrons. Na⁺ (11 − 1 = 10) and F⁻ (9 + 1 = 10) both have 10 electrons, so both are isoelectronic with neon.

Question 32General and Physical Chemistry

In the reaction Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s), which species is oxidised?

  1. Cu
  2. Zn Correct answer.
  3. Zn²⁺
  4. Cu²⁺

Explanation

Zinc loses two electrons, increasing its oxidation number from 0 to +2, so it is oxidised. Copper ion gains electrons and is reduced.

Question 33General and Physical Chemistry

Which type of isomerism is shown by propanal and propanone?

  1. Position isomerism
  2. Functional group isomerism Correct answer.
  3. Geometrical isomerism
  4. Chain isomerism

Explanation

Both compounds share the molecular formula C₃H₆O but contain different functional groups, an aldehyde and a ketone. That makes them functional group isomers.

Question 34General and Physical Chemistry

Equal masses of hydrogen gas and oxygen gas are mixed in a container at the same temperature. Which statement about the two gases is correct?

  1. Both gases exert equal partial pressures.
  2. Both gases have equal numbers of molecules.
  3. Oxygen has more molecules than hydrogen.
  4. Hydrogen has more molecules than oxygen. Correct answer.

Explanation

Hydrogen has a molar mass of about 2 g/mol and oxygen about 32 g/mol, so equal masses contain far more moles of hydrogen. More moles means more molecules and a larger mole fraction, hence a higher partial pressure.

Question 35General and Physical Chemistry

A gaseous hydrocarbon contains 85.7% carbon and 14.3% hydrogen by mass. What is its empirical formula?

  1. CH₂ Correct answer.
  2. C₂H₄
  3. CH₃
  4. C₂H₆

Explanation

Dividing percentages by atomic masses gives C = 85.7/12 ≈ 7.14 and H = 14.3/1 = 14.3. Dividing by the smaller value gives a ratio of 1:2, so the empirical formula is CH₂.

Question 36General and Physical Chemistry

For a reaction, ΔH = −120 kJ mol⁻¹ and ΔS = −300 J K⁻¹ mol⁻¹. At what temperature does the reaction become spontaneous?

  1. Below 400 K Correct answer.
  2. At all temperatures
  3. At no temperature
  4. Above 400 K

Explanation

Spontaneity requires ΔG = ΔH − TΔS < 0. With both ΔH and ΔS negative, the reaction is spontaneous when TΔS is less negative than ΔH, that is when T < ΔH/ΔS = 120000/300 = 400 K.

Question 37General and Physical Chemistry

A first-order reaction has a half-life of 20 minutes. What fraction of the reactant remains after 60 minutes?

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

Explanation

Sixty minutes corresponds to three half-lives (60/20 = 3). Each half-life halves the amount, so the remaining fraction is (1/2)³ = 1/8.

Question 38General and Physical Chemistry

For the equilibrium N₂O₄(g) ⇌ 2NO₂(g), what happens to the value of Kp when the total pressure is increased at constant temperature?

  1. Kp decreases.
  2. Kp remains unchanged. Correct answer.
  3. Kp first increases then decreases.
  4. Kp increases.

Explanation

The equilibrium constant depends only on temperature for a given reaction. Changing pressure shifts the position of equilibrium but does not change the value of Kp.

Question 39General and Physical Chemistry

The standard electrode potential of Ag⁺/Ag is +0.80 V and that of Fe³⁺/Fe²⁺ is +0.77 V. Which statement about the cell Ag | Ag⁺ || Fe³⁺, Fe²⁺ is correct?

  1. Ag is the anode and the cell EMF is +0.03 V. Correct answer.
  2. Ag is the cathode and the cell EMF is +0.03 V.
  3. Ag is the anode and the cell EMF is −0.03 V.
  4. Ag is the cathode and the cell EMF is +1.57 V.

Explanation

The species with the more positive reduction potential is reduced, so Ag⁺ is reduced at the silver electrode, making silver the cathode of the spontaneous cell written the other way. When written as Ag | Ag⁺ || Fe³⁺, Fe²⁺, silver is oxidised at the anode, and EMF = 0.77 − 0.80 = −0.03 V for that orientation. Since the question asks about the cell with Ag on the left as anode, the standard cell reaction has Ag oxidised and EMF = +0.03 V for the reverse spontaneous direction, so Ag acts as anode with a small positive driving force in the spontaneous sense.

Question 40General and Physical Chemistry

Which statement about the solubility of AgCl in water is correct when a small amount of NaCl is added?

  1. Solubility of AgCl increases because of complex formation.
  2. Solubility of AgCl increases because ionic strength rises.
  3. Solubility of AgCl is unaffected because Ksp is constant.
  4. Solubility of AgCl decreases because of the common ion effect. Correct answer.

Explanation

Adding NaCl increases [Cl⁻], and by Le Chatelier's principle the dissolution equilibrium AgCl(s) ⇌ Ag⁺ + Cl⁻ shifts left. This common ion effect lowers the solubility of AgCl.

Question 41General and Physical Chemistry

Which quantum number determines the orientation in space of an atomic orbital?

  1. Principal quantum number
  2. Spin quantum number
  3. Azimuthal quantum number
  4. Magnetic quantum number Correct answer.

Explanation

The magnetic quantum number mℓ specifies the spatial orientation of an orbital within a given subshell. The principal quantum number gives the shell, the azimuthal gives the subshell shape, and the spin describes electron spin.

Question 42General and Physical Chemistry

Which compound shows cis–trans isomerism?

  1. But-1-ene
  2. But-2-ene Correct answer.
  3. 2-methylpropene
  4. Propene

Explanation

Cis–trans isomerism requires a carbon–carbon double bond with two different groups on each doubly bonded carbon. But-2-ene, CH₃CH=CHCH₃, satisfies this, giving distinct cis and trans forms.

Question 43General and Physical Chemistry

Which factor increases the rate of an enzyme-catalysed reaction up to a limiting point?

  1. Increasing substrate concentration Correct answer.
  2. Adding a heavy-metal ion
  3. Cooling the reaction far below the optimum
  4. Removing all water from the medium

Explanation

Raising substrate concentration increases the frequency of enzyme–substrate collisions, raising the rate until the enzyme becomes saturated. Beyond that point the rate levels off.

Question 44General and Physical Chemistry

In the reaction 2KMnO₄ + 16HCl → 2KCl + 2MnCl₂ + 5Cl₂ + 8H₂O, what is the change in oxidation number of manganese?

  1. From +7 to +2 Correct answer.
  2. From +6 to +2
  3. From +7 to +4
  4. From +4 to +2

Explanation

In permanganate the manganese has oxidation number +7, and in MnCl₂ it is +2. Manganese is therefore reduced by five electrons per atom.

Question 45General and Physical Chemistry

Which statement explains why the first ionisation energy of magnesium is higher than that of sodium?

  1. Magnesium has a greater nuclear charge while the electron removed comes from the same shell. Correct answer.
  2. Sodium has a noble-gas electron configuration.
  3. Magnesium has a larger atomic radius than sodium.
  4. Magnesium has a full 3s subshell.

Explanation

The outer electron of both elements is in the 3s orbital, but magnesium has one more proton, so the effective nuclear charge is greater and the electron is held more strongly.

Question 46General and Physical Chemistry

A solution contains 0.10 M acetic acid (Ka = 1.8 × 10⁻⁵) and 0.10 M sodium acetate. What is the approximate pH of this buffer?

  1. 4.74 Correct answer.
  2. 9.26
  3. 2.87
  4. 7.00

Explanation

For a buffer, pH = pKa + log([salt]/[acid]). Here the acid and salt concentrations are equal, so pH = pKa = −log(1.8 × 10⁻⁵) ≈ 4.74.

Question 47General and Physical Chemistry

Which statement about the adsorption of a gas on a solid surface is consistent with the Langmuir isotherm?

  1. Adsorption occurs only in multilayers at low pressure.
  2. Adsorption is independent of pressure at all pressures.
  3. Adsorption reaches a limiting monolayer coverage at high pressure. Correct answer.
  4. Adsorption increases without limit as pressure rises.

Explanation

The Langmuir model assumes a fixed number of equivalent sites and single-layer adsorption, so coverage approaches a maximum as pressure increases. At high pressure the surface becomes saturated.

Question 48General and Physical Chemistry

For the reaction A + B → C, doubling [A] leaves the rate unchanged while doubling [B] doubles the rate. Which rate law is consistent with these observations?

  1. Rate = k[A]²[B]
  2. Rate = k[A][B]
  3. Rate = k[B] Correct answer.
  4. Rate = k[A]

Explanation

Independence from [A] means the order with respect to A is zero, while doubling [B] doubling the rate means the order with respect to B is one. The rate law is therefore first order overall in B only.

Question 49General and Physical Chemistry

For the galvanic cell Zn | Zn²⁺(1 M) || Cu²⁺(1 M) | Cu with E°cell = 1.10 V, which change increases the measured cell potential?

  1. Increasing both concentrations equally
  2. Increasing [Zn²⁺]
  3. Decreasing [Zn²⁺] Correct answer.
  4. Decreasing [Cu²⁺]

Explanation

By the Nernst equation, E = E° − (0.059/2)log([Zn²⁺]/[Cu²⁺]). Decreasing [Zn²⁺] lowers the log term, so the cell potential rises. Reducing the product concentration of the anodic half-reaction favours oxidation and increases EMF.

Question 50General and Physical Chemistry

An element crystallises in a face-centred cubic lattice with edge length a. What is the relation between the radius r of an atom and a?

  1. r = a√3/4
  2. r = a√2
  3. r = a√2/4 Correct answer.
  4. r = a/2

Explanation

In an fcc unit cell, atoms touch along the face diagonal, which has length a√2. The face diagonal equals four atomic radii, so 4r = a√2 and r = a√2/4.