I85PREP
⌘ K
PRC Chemist study materials
PRC Chemist study resources

PRC Chemist practice questions

One question at a time. Build your PRC Chemist understanding.

First 50 of 200 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1General Chemistry

Which statement correctly describes the ground-state electron configuration of a neutral chromium atom (Z = 24)?

  1. [Ar] 3d⁶
  2. [Ar] 4s² 3d⁴
  3. [Ar] 4s² 3d⁶
  4. [Ar] 4s¹ 3d⁵ Correct answer.

Explanation

Half-filled subshells gain extra exchange stabilization, so one 4s electron moves into 3d, giving [Ar] 4s¹ 3d⁵. This configuration has six unpaired electrons and is experimentally confirmed for gaseous Cr. The Aufbau-only prediction [Ar] 4s² 3d⁴ is close in energy but not the ground state.

Question 2General Chemistry

A sample of an ideal gas occupies 2.00 L at 300 K and 1.00 atm. It is heated to 600 K at constant pressure. What volume does it occupy?

  1. 1.00 L
  2. 8.00 L
  3. 4.00 L Correct answer.
  4. 2.00 L

Explanation

Charles's law states that volume is directly proportional to absolute temperature at constant pressure and amount. Doubling the kelvin temperature from 300 K to 600 K therefore doubles the volume from 2.00 L to 4.00 L.

Question 3General Chemistry

Which quantity is conserved in every chemical reaction in a closed system?

  1. Total mass of the system Correct answer.
  2. Number of moles of gas
  3. Total volume of the system
  4. Total number of molecules

Explanation

The law of conservation of mass states that atoms are neither created nor destroyed in a chemical reaction, so total mass is unchanged in a closed system. Moles of gas, molecule counts, and volume can all change as reactants rearrange into products.

Question 4General Chemistry

Which pair of species constitutes a conjugate acid–base pair according to Brønsted–Lowry theory?

  1. HCl and NaOH
  2. NH₄⁺ and NH₃ Correct answer.
  3. H₃O⁺ and OH⁻
  4. H₂SO₄ and SO₄²⁻

Explanation

A conjugate acid–base pair differs by exactly one proton. NH₄⁺ donates a proton to become NH₃, so the two form a conjugate pair.

Question 5General Chemistry

What is the oxidation state of manganese in the permanganate ion, MnO₄⁻?

  1. +2
  2. +4
  3. +7 Correct answer.
  4. +8

Explanation

Each oxygen is assigned −2, giving a total of −8 from the four oxygens. Since the ion carries a −1 charge, manganese must be +7 so that +7 + (−8) = −1.

Question 6General Chemistry

Which intermolecular force is primarily responsible for the unusually high boiling point of water compared with hydrogen sulfide?

  1. Hydrogen bonding Correct answer.
  2. London dispersion forces
  3. Ion–dipole interactions
  4. Dipole–dipole interactions

Explanation

Water molecules form extensive hydrogen bonds because hydrogen is bonded to a highly electronegative oxygen with lone pairs available. H₂S has weaker hydrogen bonding at best and much weaker dipole interactions, so it boils far lower.

Question 7General Chemistry

In an electrolytic cell, which process occurs at the cathode?

  1. Oxidation, with electrons leaving the electrode
  2. Reduction, with electrons supplied to the species being reduced Correct answer.
  3. Reduction, with electrons removed from the species being reduced
  4. Oxidation, with electrons supplied to the electrode

Explanation

By definition the cathode is the electrode where reduction occurs, regardless of whether the cell is galvanic or electrolytic. In an electrolytic cell an external power source pushes electrons to the cathode, where a species gains them.

Question 8General Chemistry

What is the pH of a 0.010 M solution of a strong monoprotic acid at 25 °C?

  1. 7.00
  2. 1.00
  3. 12.00
  4. 2.00 Correct answer.

Explanation

A strong acid dissociates completely, so [H₃O⁺] equals the analytical concentration, 0.010 M. The pH is −log(0.010), which equals 2.00.

Question 9General Chemistry

Which quantum number determines the overall shape of an atomic orbital (s, p, d, or f)?

  1. Magnetic quantum number mₗ
  2. Angular momentum quantum number l Correct answer.
  3. Principal quantum number n
  4. Spin quantum number mₛ

Explanation

The angular momentum quantum number l takes values from 0 to n−1 and defines the subshell type, which corresponds to orbital shape. The principal quantum number sets size and energy, mₗ sets orientation, and mₛ describes electron spin.

Question 10General Chemistry

A reaction has ΔH = −120 kJ and ΔS = −300 J/K at 298 K. Which statement about spontaneity is correct?

  1. The reaction is spontaneous at all temperatures.
  2. The reaction is spontaneous at 298 K but becomes nonspontaneous at high temperature. Correct answer.
  3. The reaction is nonspontaneous at 298 K but becomes spontaneous at high temperature.
  4. The reaction is nonspontaneous at all temperatures.

Explanation

ΔG = ΔH − TΔS. At 298 K, TΔS = −89.4 kJ, so ΔG = −120 + 89.4 = −30.6 kJ, which is negative and spontaneous. As temperature rises, the −TΔS term becomes more positive, eventually making ΔG positive, so the reaction becomes nonspontaneous at high temperature.

Question 11General Chemistry

How many significant figures should be reported in the result of the calculation 12.34 × 0.0021?

  1. Six
  2. Two Correct answer.
  3. Three
  4. Four

Explanation

In multiplication, the result carries the same number of significant figures as the factor with the fewest. The value 0.0021 has two significant figures, so the product (0.0259) is reported to two significant figures, 0.026.

Question 12General Chemistry

Which compound is expected to have the highest lattice energy?

  1. CaO
  2. KCl
  3. NaCl
  4. MgO Correct answer.

Explanation

Lattice energy increases with the product of ionic charges and decreases with ionic radius. MgO contains 2+ and 2− ions, giving a much larger charge product than the 1+ and 1− ions in NaCl or KCl, and its ions are smaller than those in CaO.

Question 13General Chemistry

Which of the following molecules is polar overall?

  1. CO₂
  2. H₂O Correct answer.
  3. BF₃
  4. CCl₄

Explanation

Water has a bent geometry, so its two O–H bond dipoles do not cancel and the molecule has a net dipole moment. CO₂, CCl₄, and BF₃ are symmetric, so their bond dipoles cancel to give zero net dipole.

Question 14General Chemistry

What is the hybridization of the central carbon atom in the carbonate ion, CO₃²⁻?

  1. sp³d
  2. sp³
  3. sp² Correct answer.
  4. sp

Explanation

The carbonate ion has three sigma bonds and one delocalized pi system around the carbon. Three electron domains correspond to sp² hybridization and a trigonal planar geometry.

Question 15General Chemistry

Which factor increases the rate of a gas-phase reaction between two molecules?

  1. Increasing the concentration of the reactants Correct answer.
  2. Decreasing the temperature
  3. Increasing the activation energy
  4. Adding a catalyst that raises the activation energy

Explanation

Higher reactant concentration increases the frequency of collisions, which raises the reaction rate. Temperature and activation energy affect the fraction of collisions with sufficient energy, and raising either activation energy or lowering temperature slows the reaction.

Question 16General Chemistry

A buffer is prepared from equal concentrations of acetic acid and sodium acetate. If a small amount of strong acid is added, what happens?

  1. The acetate ion consumes added protons, and the pH changes only slightly. Correct answer.
  2. The acetic acid consumes added protons, and the pH changes only slightly.
  3. The pH rises sharply because strong acid neutralizes the acetate.
  4. The pH drops sharply because the buffer is destroyed immediately.

Explanation

In an acetic acid/acetate buffer, added strong acid is neutralized by the conjugate base, converting acetate into acetic acid. The ratio of acid to base changes only slightly, so the pH changes only slightly according to the Henderson–Hasselbalch equation.

Question 17General Chemistry

Which statement about a reaction at equilibrium is correct?

  1. The concentrations of reactants and products are equal.
  2. The equilibrium constant is exactly 1 under all conditions.
  3. The forward and reverse reaction rates are equal and nonzero. Correct answer.
  4. The reaction has stopped completely in both directions.

Explanation

At equilibrium the forward and reverse rates are equal, so concentrations remain constant while both reactions continue on the molecular level. Concentrations need not be equal, and the equilibrium constant depends on the specific reaction and temperature.

Question 18General Chemistry

Which of the following is a state function?

  1. Path length of the process
  2. Heat q
  3. Enthalpy H Correct answer.
  4. Work w

Explanation

A state function depends only on the current state of the system, not on the path taken. Enthalpy, internal energy, and entropy are state functions, whereas heat and work depend on the path and are not.

Question 19General Chemistry

Which pair of elements is most likely to form an ionic compound?

  1. Two noble gases
  2. Two halogens
  3. Two nonmetals with similar electronegativities
  4. A metal with low electronegativity and a nonmetal with high electronegativity Correct answer.

Explanation

Ionic bonding typically occurs between a metal that readily loses electrons and a nonmetal that readily gains them. A large electronegativity difference favors electron transfer rather than sharing.

Question 20General Chemistry

A 25.0 mL sample of 0.100 M NaOH is titrated with 0.100 M HCl. What volume of HCl is required to reach the equivalence point?

  1. 25.0 mL Correct answer.
  2. 100.0 mL
  3. 12.5 mL
  4. 50.0 mL

Explanation

NaOH and HCl react in a 1:1 mole ratio. The moles of NaOH are 0.0250 L × 0.100 M = 2.50 × 10⁻³ mol, so 2.50 × 10⁻³ mol of HCl is needed. Dividing by 0.100 M gives 0.0250 L, or 25.0 mL.

Question 21General Chemistry

Which element has the largest atomic radius?

  1. Rb Correct answer.
  2. Na
  3. Li
  4. K

Explanation

Atomic radius increases down a group because each period adds a new principal shell, moving valence electrons farther from the nucleus. Among the alkali metals listed, rubidium lies lowest in the group and therefore has the largest radius.

Question 22General Chemistry

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

  1. n = 2, l = 1, mₗ = 0, mₛ = +½
  2. n = 1, l = 1, mₗ = 0, mₛ = +½ Correct answer.
  3. n = 3, l = 2, mₗ = −2, mₛ = −½
  4. n = 4, l = 0, mₗ = 0, mₛ = −½

Explanation

The angular momentum quantum number l must satisfy 0 ≤ l ≤ n − 1. For n = 1, only l = 0 is allowed, so l = 1 is forbidden. The other sets satisfy all constraints on n, l, mₗ, and mₛ.

Question 23General Chemistry

Which compound is named correctly according to IUPAC nomenclature?

  1. 3-methylbutane for CH₃CH(CH₃)CH₂CH₃
  2. 2-methylpropane for CH₃CH(CH₃)CH₃ Correct answer.
  3. 1-methylpropane for CH₃CH₂CH₂CH₃
  4. 2,2-dimethylpropane for CH₃CH₂CH(CH₃)₂

Explanation

The longest chain in CH₃CH(CH₃)CH₃ has three carbons, with a methyl group on the middle carbon, giving 2-methylpropane. Substituent locants must be as low as possible, and the parent chain must be the longest continuous carbon chain.

Question 24General Chemistry

Consider the equilibrium N₂(g) + 3H₂(g) ⇌ 2NH₃(g), which is exothermic. Which change shifts the equilibrium toward more NH₃?

  1. Decreasing the total pressure
  2. Increasing the temperature
  3. Adding a catalyst
  4. Removing NH₃ as it forms Correct answer.

Explanation

Removing a product lowers its concentration and drives the reaction forward according to Le Chatelier's principle. For this exothermic reaction, raising temperature shifts the equilibrium back toward reactants, and lowering pressure favors the side with more gas moles, which is the reactant side.

Question 25General Chemistry

A galvanic cell is constructed from a Zn²⁺/Zn half-cell (E° = −0.76 V) and an Ag⁺/Ag half-cell (E° = +0.80 V). What is the standard cell potential?

  1. −1.56 V
  2. +1.56 V Correct answer.
  3. +0.04 V
  4. +0.80 V

Explanation

In a galvanic cell the more positive reduction potential is the cathode, so silver is reduced and zinc is oxidized. E°cell = E°cathode − E°anode = 0.80 − (−0.76) = +1.56 V, which is positive as required for a spontaneous galvanic cell.

Question 26General Chemistry

Which of the following is an example of a heterogeneous mixture?

  1. A sugar solution in water
  2. A sodium chloride solution in water
  3. A brass alloy
  4. A mixture of sand and iron filings Correct answer.

Explanation

A heterogeneous mixture has a composition that is not uniform throughout, and its components can be visibly distinguished. Sand and iron filings remain physically distinct phases and can be separated by a magnet, so this is heterogeneous. Solutions such as sugar water, salt water, and brass are uniform throughout and are homogeneous.

Question 27General Chemistry

A sample of a gas occupies 2.00 L at 300 K. Assuming constant pressure, what volume does it occupy at 600 K?

  1. 4.00 L Correct answer.
  2. 2.00 L
  3. 8.00 L
  4. 1.00 L

Explanation

Charles's law states that at constant pressure the volume of a fixed amount of gas is directly proportional to its absolute temperature (V₁/T₁ = V₂/T₂). Doubling the temperature from 300 K to 600 K therefore doubles the volume from 2.00 L to 4.00 L.

Question 28General Chemistry

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

  1. 1s² 2s² 2p⁶ 3s² 3p⁴
  2. 1s² 2s² 2p⁶ 3s² 3p⁵ Correct answer.
  3. 1s² 2s² 2p⁵ 3s² 3p⁶
  4. 1s² 2s² 2p⁶ 3s² 3p⁶

Explanation

A neutral chlorine atom has 17 electrons. Filling orbitals in order of increasing energy gives 1s² 2s² 2p⁶ 3s² 3p⁵, which totals 2 + 2 + 6 + 2 + 5 = 17 electrons and leaves one vacancy in the 3p subshell.

Question 29General Chemistry

A 0.50 mol sample of an ideal gas is held at 2.0 atm in a 6.0 L container. Using R = 0.0821 L·atm/(mol·K), what is the approximate temperature of the gas?

  1. 292 K Correct answer.
  2. 1170 K
  3. 146 K
  4. 584 K

Explanation

Rearranging the ideal gas law gives T = PV/(nR). Substituting P = 2.0 atm, V = 6.0 L, n = 0.50 mol, and R = 0.0821 gives T = (2.0 × 6.0)/(0.50 × 0.0821) ≈ 12/0.04105 ≈ 292 K.

Question 30General Chemistry

Which of the following sets of quantum numbers (n, l, mₗ, mₛ) describes an electron in a 3d orbital?

  1. 4, 3, 0, +½
  2. 3, 2, −2, −½ Correct answer.
  3. 3, 1, 0, +½
  4. 2, 2, 1, +½

Explanation

A 3d orbital has principal quantum number n = 3 and azimuthal quantum number l = 2, with mₗ ranging from −2 to +2. The set (3, 2, −2, −½) satisfies all constraints and identifies a 3d orbital.

Question 31General Chemistry

Which of the following compounds contains both ionic and covalent bonding?

  1. Sodium chloride, NaCl
  2. Methane, CH₄
  3. Potassium nitrate, KNO₃ Correct answer.
  4. Carbon dioxide, CO₂

Explanation

Potassium nitrate contains ionic bonding between K⁺ and the polyatomic nitrate ion, and covalent bonding between nitrogen and oxygen within the nitrate ion itself. Compounds that combine a metal cation with a polyatomic ion therefore exhibit both bonding types.

Question 32General Chemistry

In an acid–base titration, 25.0 mL of a 0.200 M monoprotic acid solution requires 20.0 mL of NaOH solution to reach the equivalence point. What is the concentration of the NaOH solution?

  1. 0.400 M
  2. 0.200 M
  3. 0.250 M Correct answer.
  4. 0.160 M

Explanation

At the equivalence point the moles of acid equal the moles of base for a monoprotic acid: M_acid × V_acid = M_base × V_base. Solving gives M_base = (0.200 M × 25.0 mL)/20.0 mL = 0.250 M.

Question 33General Chemistry

Consider the reaction 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), which is exothermic in the forward direction. Which change would increase the equilibrium yield of SO₃?

  1. Increasing the temperature
  2. Increasing the total pressure Correct answer.
  3. Removing O₂ from the mixture
  4. Decreasing the total pressure

Explanation

The forward reaction converts three moles of gas into two moles of gas, so increasing pressure favors the side with fewer gas molecules, increasing SO₃ yield. Le Chatelier's principle predicts that compression shifts this equilibrium to the right.

Question 34General Chemistry

A galvanic cell is based on the half-reactions Cu²⁺ + 2e⁻ → Cu (E° = +0.34 V) and Fe²⁺ + 2e⁻ → Fe (E° = −0.44 V). What is the standard cell potential, and which electrode is the cathode?

  1. +0.78 V, with copper as the cathode Correct answer.
  2. −0.78 V, with iron as the cathode
  3. +0.10 V, with copper as the cathode
  4. +0.78 V, with iron as the cathode

Explanation

The standard cell potential equals the more positive reduction potential minus the less positive one: E°cell = 0.34 V − (−0.44 V) = +0.78 V. The electrode with the higher reduction potential, copper, undergoes reduction and is therefore the cathode.

Question 35Analytical Chemistry

In a flame atomic absorption spectrometer, what is the primary function of the hollow cathode lamp?

  1. It disperses transmitted light into its component wavelengths
  2. It atomizes the sample into a ground-state vapor
  3. It detects the absorbance signal and converts it to a voltage
  4. It emits a narrow line spectrum of the analyte element used as the primary source Correct answer.

Explanation

The hollow cathode lamp contains a cathode made of the analyte element, so it emits the element's characteristic narrow emission lines. Matching the source to the analyte line gives the selectivity and Beer-Lambert behavior that atomic absorption depends on. Atomization, dispersion, and detection are performed by other components.

Question 36Analytical Chemistry

What does the term 'reversed-phase' mean in partition chromatography?

  1. The elution order of analytes is reversed relative to size
  2. The stationary phase is nonpolar and the mobile phase is polar Correct answer.
  3. The stationary phase is polar and the mobile phase is nonpolar
  4. The column is operated with flow in the reverse direction

Explanation

Reversed-phase chromatography uses a nonpolar stationary phase such as a C18 hydrocarbon layer with a polar mobile phase like water-methanol or water-acetonitrile. This is the reverse of classical normal-phase systems, where silica is polar and the mobile phase is nonpolar. Retention increases with analyte hydrophobicity.

Question 37Analytical Chemistry

In potentiometric pH measurement, the potential of the glass electrode responds to which quantity?

  1. Total dissolved solids in the sample
  2. Conductivity of the internal reference solution
  3. Hydrogen ion activity in the sample solution Correct answer.
  4. Oxidation-reduction potential of the sample

Explanation

The glass membrane develops a potential that varies with the activity of hydrogen ions according to the Nernst equation, roughly 59 mV per decade at 25 degrees C. The meter converts this potential difference against the reference electrode into a pH reading. Conductivity, TDS, and redox potential are measured by other sensors.

Question 38Analytical Chemistry

What is the main advantage of a double-beam UV-visible spectrophotometer over a single-beam instrument?

  1. It increases the path length of the sample cell
  2. It eliminates the need for a monochromator
  3. It allows measurement of emission spectra
  4. It compensates for source drift and solvent absorbance by measuring reference and sample simultaneously Correct answer.

Explanation

A double-beam design splits light between a reference channel and the sample channel, so fluctuations in lamp intensity and solvent background are cancelled in real time. This improves stability for long scans and baseline flatness. Path length, dispersion, and emission measurement are unaffected by beam configuration.

Question 39Analytical Chemistry

A sample is analyzed by gas chromatography and the peak areas are measured. Which statement correctly describes an internal standard?

  1. The detector response is normalized to the total peak area
  2. A known amount of the analyte itself is added to the sample before analysis
  3. A known amount of a compound absent from the sample is added and the analyte response is ratioed to it Correct answer.
  4. A calibration curve is constructed from pure solvent blanks

Explanation

An internal standard is a substance not originally present in the sample, added in known amount, whose response corrects for injection variability and matrix effects. The ratio of analyte peak area to internal standard peak area is used for quantitation. Adding the analyte itself is standard addition, a different technique.

Question 40Analytical Chemistry

In complexometric titration with EDTA, why is a buffer such as ammonia-ammonium chloride often added?

  1. To act as a redox indicator for the endpoint
  2. To maintain a pH at which the conditional formation constant is favorable and metal hydrolysis is minimized Correct answer.
  3. To precipitate interfering sulfate ions
  4. To increase the solubility of EDTA in water

Explanation

EDTA complexation depends strongly on pH because protonation of the carboxylate groups competes with metal binding. Buffering to an appropriate pH maximizes the conditional formation constant while preventing metal hydroxide precipitation. EDTA is already water-soluble, and buffers are not redox indicators.

Question 41Analytical Chemistry

Which detector is most commonly used in gas chromatography because of its broad response to organic compounds and good sensitivity?

  1. Refractive index detector
  2. Thermal conductivity detector
  3. Conductivity detector
  4. Flame ionization detector Correct answer.

Explanation

The flame ionization detector ionizes organic molecules in a hydrogen-air flame and produces a current proportional to carbon content. It responds to nearly all organic compounds with high sensitivity. Refractive index and conductivity detectors are used in liquid-phase systems, and thermal conductivity detectors are less sensitive.

Question 42Analytical Chemistry

In gravimetric analysis, what does 'constant weight' mean when drying a precipitate?

  1. The balance is calibrated to a certified reference mass
  2. The crucible mass equals the precipitate mass
  3. Successive drying and weighing steps agree within a small specified tolerance Correct answer.
  4. The precipitate is weighed exactly once after filtration

Explanation

Constant weight is reached when repeated drying and cooling cycles give masses that differ by no more than a defined small amount, typically 0.2 to 0.3 mg. This confirms that residual solvent or volatile material has been removed. It is not a single weighing or a calibration check.

Question 43Analytical Chemistry

A 0.2500 g sample of a chloride-containing salt is titrated with 0.1000 M silver nitrate, consuming 24.50 mL to the endpoint. What is the mass percent of chloride in the sample?

  1. 17.38%
  2. 69.50%
  3. 24.50%
  4. 34.75% Correct answer.

Explanation

Moles of silver nitrate equal 0.1000 mol/L times 0.02450 L, which is 0.002450 mol, and this equals moles of chloride by the 1:1 stoichiometry. Multiplying by 35.45 g/mol gives 0.08685 g of chloride. Dividing by 0.2500 g and multiplying by 100 gives 34.75 percent.

Question 44Analytical Chemistry

A solution has a transmittance of 25.0% in a 1.00 cm cell. What is its absorbance?

  1. 0.398
  2. 1.602
  3. 0.602 Correct answer.
  4. 0.250

Explanation

Absorbance equals the negative logarithm of transmittance expressed as a fraction, so A = -log(0.250). The logarithm of 0.250 is about -0.602, giving an absorbance of 0.602. Absorbance is dimensionless and is not the same as percent transmittance.

Question 45Analytical Chemistry

In high-performance liquid chromatography, what is the consequence of reducing particle size in the stationary phase from 10 micrometers to 3 micrometers at constant linear velocity?

  1. Column back pressure decreases proportionally
  2. Efficiency increases because the mass-transfer and eddy-diffusion contributions to plate height decrease Correct answer.
  3. Efficiency decreases because longitudinal diffusion becomes dominant
  4. Resolution is unaffected because selectivity depends only on the mobile phase

Explanation

Smaller particles shorten the diffusion distances in the mobile and stationary phases and reduce eddy diffusion, lowering the plate height and increasing efficiency. The trade-off is a substantial rise in back pressure, which is why higher-pressure pumps are needed. Selectivity is largely unchanged, and resolution usually improves.

Question 46Analytical Chemistry

Which statement correctly describes the role of the matrix modifier in graphite furnace atomic absorption spectroscopy?

  1. It removes the need for background correction
  2. It stabilizes the analyte to allow higher ashing temperatures or volatilizes the matrix Correct answer.
  3. It increases the atomization temperature of the graphite tube
  4. It serves as the primary light source for absorption

Explanation

A matrix modifier such as magnesium nitrate or palladium alters the thermal behavior of the sample so the analyte is retained during ashing while matrix components are removed. This reduces interference and loss of volatile analytes. It does not replace the light source or background correction.

Question 47Analytical Chemistry

What is the purpose of suppressing the electroosmotic flow in capillary zone electrophoresis when measuring the electrophoretic mobility of an anion?

  1. To allow the anion to migrate toward the detector by its own charge rather than being swept backward Correct answer.
  2. To increase the capillary length
  3. To prevent Joule heating in the capillary
  4. To increase the ionic strength of the buffer

Explanation

Electroosmotic flow normally sweeps bulk solution toward the cathode, which can oppose the migration of anions toward the detector. Suppressing or reversing EOF lets the intrinsic electrophoretic mobility of the anion control migration direction and speed. Joule heating and ionic strength are controlled separately.

Question 48Analytical Chemistry

In X-ray fluorescence spectrometry, what is the origin of characteristic X-rays emitted from a sample?

  1. Inner-shell electron transitions following ionization of atoms by the primary beam Correct answer.
  2. Nuclear decay of radioactive isotopes in the sample
  3. Raman scattering of the incident X-ray photons
  4. Thermal emission from the sample heated by the X-ray tube

Explanation

The primary X-ray beam ejects inner-shell electrons, and outer-shell electrons fall into the vacancies, emitting photons with energies characteristic of the element. The energies identify the elements and the intensities relate to concentration. Nuclear decay and thermal emission are unrelated processes.

Question 49Analytical Chemistry

A calibration curve for lead by ICP-OES gives a slope of 0.0450 (signal per mg/L) and an intercept of 0.0020. A sample gives a signal of 0.2270. What is the lead concentration?

  1. 5.04 mg/L
  2. 0.0050 mg/L
  3. 0.2270 mg/L
  4. 5.00 mg/L Correct answer.

Explanation

Subtract the intercept from the sample signal to get 0.2250, then divide by the slope of 0.0450 to obtain 5.00 mg/L. The intercept correction removes the small baseline offset. Reporting the raw signal or dividing by the wrong factor gives incorrect values.

Question 50Analytical Chemistry

Which technique separates analytes primarily on the basis of molecular size in solution?

  1. Chiral chromatography
  2. Size-exclusion chromatography Correct answer.
  3. Ion-exchange chromatography
  4. Affinity chromatography

Explanation

Size-exclusion chromatography uses a porous stationary phase in which small molecules penetrate the pores and are retarded, while large molecules are excluded and elute first. Separation depends on hydrodynamic volume rather than charge or specific binding. Ion exchange, affinity, and chiral separations depend on other interactions.