All Free Chemistry MCQs with Answers

• 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…

• 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…

• 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…

• 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…

• 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.

• 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…

• 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.

• 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…

• 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…

• 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.

• A. 6.02 x 10^23
• B. 4.21 x 10^24
• C. 4.214 x 10^24
• D. 98

Explanation: One molecule contains 2 + 1 + 4, that is 7 atoms, so one mole contains 7 x 6.02 x 10^23, which is 4.214 x 10^24 atoms.

• A. CH2O
• B. C2H4O2
• C. C3H6O3
• D. C6H12O6

Explanation: The empirical formula mass is 12 + 2 + 16, that is 30, and 180 divided by 30 gives 6, so every subscript is multiplied by six to give…

• A. 27.3 per cent
• B. 12 per cent
• C. 44 per cent
• D. 72.7 per cent

Explanation: The molar mass is 12 + 32, that is 44, so the carbon fraction is 12 divided by 44 multiplied by 100, giving 27.3 per cent.

• A. is present in the largest amount by mass
• B. is completely used up first and so determines the amount of product formed
• C. has the highest molar mass
• D. remains unreacted at the end

Explanation: The reaction stops when one reactant runs out, so that reactant fixes the maximum yield no matter how much of the others is present.

• A. nitrogen, because it has the larger molar mass
• B. neither, because both are used up exactly
• C. hydrogen, because 3 moles would be needed to react with 1 mole of nitrogen
• D. nitrogen, because fewer moles of it are present

Explanation: The equation demands three moles of hydrogen for every mole of nitrogen, so 1 mole of nitrogen would need 3 moles of hydrogen and only 2…

• A. 2 moles of hydrogen
• B. 3 moles of oxygen
• C. nothing, both are exactly consumed
• D. 1 mole of oxygen

Explanation: Four moles of hydrogen require only two moles of oxygen, so hydrogen is limiting and 3 minus 2, that is 1 mole of oxygen, remains…

• A. the maximum mass of product calculated from the balanced equation and the limiting reactant
• B. the mass of product actually obtained in the laboratory
• C. always equal to the mass of the limiting reactant
• D. the mass of the excess reactant left over

Explanation: Theoretical yield is a calculation, assuming the reaction goes to completion with no losses and no side reactions, and it is the standard…

• A. 125 per cent
• B. 80 per cent
• C. 5 per cent
• D. 20 per cent

Explanation: Percentage yield is actual divided by theoretical multiplied by 100, so 20 divided by 25 multiplied by 100 gives 80 per cent.

• A. some product is lost during filtration and transfer
• B. side reactions produce unwanted products
• C. the balanced equation was written incorrectly
• D. the reaction is reversible and does not go to completion

Explanation: Physical losses, competing side reactions and reversibility are all genuine reasons why a real yield falls short.

• A. 100 g
• B. 44 g
• C. 40 g
• D. 56 g

Explanation: The equation shows a one to one ratio, so 1 mole of CaCO3 gives 1 mole of CaO, whose molar mass is 40 + 16, that is 56 g.