Chemistry ยท WCH11 (Unit 1)

Amount of substance: the mole

Chemists count particles in moles. One mole contains 6.02 x 1023 particles (the Avogadro constant).

Moles from mass

amount (mol) = mass (g) / molar mass (g/mol). Molar mass is the relative formula mass in g/mol: for CO2, 12.0 + 2(16.0) = 44.0 g/mol.

Moles in solution

amount (mol) = concentration (mol/dm3) x volume (dm3). Convert cm3 to dm3 by dividing by 1000.

Using equations

Balanced equations give mole ratios. In 2H2 + O2 -> 2H2O, 2 mol of hydrogen react with 1 mol of oxygen to make 2 mol of water.

Worked example

How many moles are in 11.0 g of CO2?

  1. M(CO2) = 12.0 + 32.0 = 44.0 g/mol
  2. n = 11.0 / 44.0

Answer: 0.250 mol

Key idea: n = m / M; n = c x V (V in dm3); use the balanced equation's mole ratio.

Check yourself

  1. How many moles are in 36.0 g of water? (H = 1.0, O = 16.0)
    Show answer

    2 mol. M = 18.0 g/mol, so n = 36.0 / 18.0 = 2.00 mol.

  2. How many moles are in 25.0 cm3 of 0.200 mol/dm3 HCl?
    Show answer

    0.005 mol. n = 0.200 x 0.0250 = 0.00500 mol.

  3. Find the molar mass of NaOH. (Na = 23.0, O = 16.0, H = 1.0)
    Show answer

    40 g/mol. 23.0 + 16.0 + 1.0 = 40.0 g/mol.

  4. In 2H2 + O2 -> 2H2O, how many moles of O2 react with 6 mol of H2?
    Show answer

    3. The ratio H2 : O2 is 2 : 1, so 6 mol H2 needs 3 mol O2.

  5. What is the mass of 0.500 mol of CaCO3? (M = 100.1 g/mol)
    Show answer

    50 g. m = n x M = 0.500 x 100.1 = 50.05 g, which is 50.1 g to 3 s.f.

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Last updated 7 November 2026. Spotted a mistake? Email alevel@lifereadysociety.com.