All Free Chemistry MCQs with Answers

41. For an endothermic reaction at equilibrium, raising the temperature will

• A. shift the equilibrium towards the products and increase Kc
• B. shift the equilibrium towards the reactants
• C. have no effect
• D. decrease the rate of both reactions

Explanation: Heat can be treated as a reactant in an endothermic reaction, so supplying more of it drives the system forward to absorb the extra energy, which is Le Chatelier's principle applied to temperature. Because the position genuinely moves rather than being restored, the value of Kc increases as well. For an exothermic reaction the same reasoning gives the opposite result.

42. A saturated solution of a sparingly soluble salt is one in which

• A. no more solid can be added to the container
• B. the dissolved ions are in equilibrium with the undissolved solid
• C. the salt has completely dissolved
• D. the solubility product has been exceeded

Explanation: In a saturated solution ions leave the solid surface and return to it at the same rate, so the concentration of dissolved ions stays constant and equals the value fixed by Ksp. If the ionic product were pushed beyond Ksp the excess would precipitate until equilibrium was restored. Undissolved solid must be present for the equilibrium to exist at all.

43. In the reaction A + B gives C + D at equilibrium, adding more of substance A will

• A. decrease the amount of C formed
• B. have no effect on the position of equilibrium
• C. shift the equilibrium to the right, forming more C and D
• D. change the value of Kc

Explanation: The system responds to the extra A by consuming some of it, which means running the forward reaction further and producing more products until the ratio required by Kc is restored. This is why one reactant is often used in excess industrially when it is the cheaper one. The value of Kc is unchanged, since only temperature can alter it.

44. The pH of a buffer solution made from a weak acid and its salt depends on

• A. the total volume of the solution
• B. the temperature only
• C. the presence of a catalyst
• D. the dissociation constant of the acid and the ratio of salt to acid concentration

Explanation: The Henderson Hasselbalch relationship gives pH as pKa plus the logarithm of the ratio of salt to acid, so it is the ratio that matters rather than the absolute amounts. Diluting the buffer changes both concentrations equally and so leaves the pH almost unaltered, which is one of its most useful features. Buffer capacity, however, does depend on how concentrated the components are.

45. The value of Avogadro's number is

• A. 6.02 x 10^-23
• B. 6.02 x 10^23
• C. 6.02 x 10^24
• D. 3.01 x 10^23

Explanation: One mole of any substance contains 6.02 x 10^23 particles, whether those particles are atoms, molecules or ions. The negative exponent in the first option would make the number vanishingly small, which is the commonest careless slip. Half that value, 3.01 x 10^23, is the number of particles in half a mole.

46. One mole of a substance is defined as

• A. the amount containing as many particles as there are atoms in 12 g of carbon 12
• B. one gram of the substance
• C. the mass of one molecule of the substance
• D. the volume occupied by one gram of a gas

Explanation: The mole is a counting unit fixed by reference to 12 g of carbon 12, which is why the molar mass of any substance in grams contains exactly Avogadro's number of particles. Defining it by mass alone would be meaningless, since one gram of hydrogen and one gram of lead contain hugely different numbers of atoms. The mole is what lets a chemist count particles by weighing.

47. The number of moles present in 36 g of water is

• A. 0.5
• B. 1
• C. 2
• D. 18

Explanation: The molar mass of water is 2(1) + 16, that is 18 g per mole, so 36 divided by 18 gives 2 moles. Those 2 moles contain 1.204 x 10^24 water molecules. The figure 18 is the molar mass itself and is offered because candidates who divide the wrong way round arrive at it.

48. The number of molecules in 2 moles of carbon dioxide is

• A. 6.02 x 10^23
• B. 3.01 x 10^23
• C. 44
• D. 1.204 x 10^24

Explanation: Two moles contain 2 x 6.02 x 10^23, which is 1.204 x 10^24 molecules, and this is true for any substance since the mole counts particles rather than mass. The number 44 is the molar mass of carbon dioxide in grams per mole, not a count of molecules. Note that the same 2 moles contain three times as many atoms, because each molecule has three.

49. The volume occupied by one mole of any ideal gas at standard temperature and pressure is

• A. 22.4 dm3
• B. 24 dm3
• C. 1 dm3
• D. 22.4 cm3

Explanation: At 273 K and 1 atmosphere one mole of any gas occupies 22.4 dm3, which follows from the ideal gas equation and is independent of the identity of the gas. The figure 24 dm3 applies at room temperature, about 298 K, so it is right for a different condition and wrong here. Reading dm3 as cm3 changes the answer by a factor of a thousand.

50. The mass of 0.5 mole of sodium hydroxide, NaOH, is

• A. 40 g
• B. 20 g
• C. 80 g
• D. 10 g

Explanation: The molar mass is 23 + 16 + 1, that is 40 g per mole, so half a mole has a mass of 20 g. Mass is calculated as number of moles multiplied by molar mass, and the commonest error is to quote the molar mass itself without applying the 0.5. This is the calculation behind preparing a standard solution in the laboratory.