Free Fundamental Concepts of Chemistry MCQs with Answers

1. 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.

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. The total number of atoms present in one mole of sulphuric acid, H2SO4, is

• 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. The value 6.02 x 10^23 is the number of molecules, not of atoms, and is the trap in this question. The number 98 is the molar mass in grams per mole.

8. A compound has the empirical formula CH2O and a relative molecular mass of 180. Its molecular formula is

• 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 C6H12O6, which is glucose. The empirical formula gives only the simplest whole number ratio of atoms, while the molecular formula gives the actual number in one molecule. The two coincide only when that multiplier happens to be one.

9. The percentage by mass of carbon in carbon dioxide, CO2, is about

• 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. The figure 72.7 per cent is the oxygen content, which is why it appears as a distractor, and the two must add to 100. Percentage composition is the first step in working out an empirical formula from experimental data.

10. The limiting reactant in a chemical reaction is the one that

• 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. It must be identified by comparing moles against the balanced equation, not by comparing masses, because a small mass of a light reactant can be many moles. The reactant left over at the end is the excess reactant.