Calculations in chemistry

JAMB Chemistry Practice Questions and Answers

1. Calculate the number of moles in 36 grams of water (H₂O).

Molar mass of H₂O = (2 × 1) + 16 = 18 g/mol.
Number of moles = mass / molar mass = 36 g / 18 g/mol = 2 moles.

2. What is the empirical formula of a compound that contains 4 g of nitrogen and 12 g of oxygen?

Moles of nitrogen = 4 g / 14 g/mol = 0.286 mol.
Moles of oxygen = 12 g / 16 g/mol = 0.75 mol.
Divide both by the smallest value (0.286):
Nitrogen: 0.286 / 0.286 = 1
Oxygen: 0.75 / 0.286 = 2.6 ≈ 3
The empirical formula is NO₃.

3. What is the molecular formula of a compound if its empirical formula is CH₂ and its molar mass is 56 g/mol?

Molar mass of CH₂ = 12 + (2 × 1) = 14 g/mol.
Ratio of molecular mass to empirical formula mass = 56 / 14 = 4.
The molecular formula is C₄H₈.

4. How many molecules are there in 2 moles of methane (CH₄)?

1 mole = 6.022 × 10²³ molecules.
Number of molecules in 2 moles = 2 × 6.022 × 10²³ = 1.2044 × 10²⁴ molecules.

5. Calculate the mass of 0.5 moles of sodium chloride (NaCl).

Molar mass of NaCl = 23 + 35.5 = 58.5 g/mol.
Mass = moles × molar mass = 0.5 × 58.5 = 29.25 grams.

6. Calculate the number of moles in 45 grams of carbon dioxide (CO₂).

Molar mass of CO₂ = 12 + (2 × 16) = 44 g/mol.
Number of moles = 45 g / 44 g/mol = 1.02 moles.

7. What is the empirical formula of a compound that contains 12 g of carbon and 32 g of oxygen?

Moles of carbon = 12 g / 12 g/mol = 1 mol.
Moles of oxygen = 32 g / 16 g/mol = 2 mol.
Divide both by the smallest value (1):
Carbon: 1 / 1 = 1
Oxygen: 2 / 1 = 2
The empirical formula is CO₂.

8. The molecular weight of a compound is 88 g/mol, and its empirical formula is CH₄. What is the molecular formula?

Molar mass of CH₄ = 12 + (4 × 1) = 16 g/mol.
Ratio of molecular mass to empirical formula mass = 88 / 16 = 5.
The molecular formula is C₅H₁₀.

9. Calculate the number of moles in 100 g of sodium (Na).

Molar mass of Na = 23 g/mol.
Number of moles = 100 g / 23 g/mol = 4.35 moles.

10. What is the mass of 0.5 moles of a substance that weighs 18 g per mole?

Mass = moles × molar mass = 0.5 × 18 = 9 grams.

11. What is the valency of oxygen in H₂O?

In H₂O, hydrogen has a valency of 1. Oxygen must balance this by having a valency of 2 to make the compound neutral.

12. How many molecules are in 10 grams of oxygen gas (O₂)?

Molar mass of O₂ = 32 g/mol.
Number of moles = 10 g / 32 g/mol = 0.3125 moles.
Number of molecules = 0.3125 × 6.022 × 10²³ = 1.88 × 10²³ molecules.

13. What is the empirical formula of a compound that contains 2.8 g of nitrogen and 8.0 g of hydrogen?

Moles of nitrogen = 2.8 g / 14 g/mol = 0.2 mol.
Moles of hydrogen = 8.0 g / 1 g/mol = 8 mol.
Divide both by the smallest value (0.2):
Nitrogen: 0.2 / 0.2 = 1
Hydrogen: 8 / 0.2 = 40
The empirical formula is NH₄.

14. What is the molecular formula of a compound with an empirical formula of CH₂ and a molar mass of 28 g/mol?

Molar mass of CH₂ = 12 + (2 × 1) = 14 g/mol.
Ratio of molecular mass to empirical formula mass = 28 / 14 = 2.
The molecular formula is C₂H₄.

15. How many grams are in 0.25 moles of potassium chloride (KCl)?

Molar mass of KCl = 39 + 35.5 = 74.5 g/mol.
Mass = 0.25 × 74.5 = 18.625 grams.

16. Calculate the number of moles in 3.2 × 10²⁴ molecules of methane (CH₄).

Number of moles = 3.2 × 10²⁴ / 6.022 × 10²³ = 5.3 moles.
Molar mass of CH₄ = 12 + (4 × 1) = 16 g/mol.
Mass = 5.3 × 16 = 84.8 grams.

17. Find the number of atoms in 4 moles of aluminum (Al).

1 mole = 6.022 × 10²³ atoms.
Number of atoms in 4 moles = 4 × 6.022 × 10²³ = 2.4088 × 10²⁴ atoms.

18. What is the empirical formula of a compound containing 2.4 g of nitrogen and 7.2 g of oxygen?

Moles of nitrogen = 2.4 g / 14 g/mol = 0.171 mol.
Moles of oxygen = 7.2 g / 16 g/mol = 0.45 mol.
Divide both by the smallest value (0.171):
Nitrogen: 0.171 / 0.171 = 1
Oxygen: 0.45 / 0.171 = 2.63 ≈ 3
The empirical formula is NO₃.

19. Calculate the number of moles in 72 grams of carbon (C).

Molar mass of C = 12 g/mol.
Number of moles = 72 g / 12 g/mol = 6 moles.

20. If 1 mole of a substance weighs 58.5 grams, how many grams will 0.2 moles weigh?

Mass = moles × molar mass = 0.2 × 58.5 = 11.7 grams.