Chemistry – Stoichiometry - The mole and the Avogadro constant | e-Consult

1. Write the balanced chemical equation:

2Mg(s) + 4HCl(aq) → 2MgCl₂(aq) + H₂(g)

2. Convert the volume of hydrogen gas to dm³:

25 cm³ = 0.025 dm³

3. Use the molar volume of a gas at r.t.p. (24 dm³/mol) to calculate the number of moles of hydrogen gas:

moles of H₂ = volume / molar volume = 0.025 dm³ / 24 dm³/mol = 0.00104 mol

4. Use the stoichiometry of the balanced equation to calculate the number of moles of magnesium that reacted:

From the equation, 2 moles of Mg react with 4 moles of H₂. Therefore, the mole ratio is 2:4 or 1:2.

moles of Mg = (1/2) * moles of H₂ = (1/2) * 0.00104 mol = 0.00052 mol

5. Calculate the mass of magnesium that reacted:

mass of Mg = moles * molar mass = 0.00052 mol * 24 g/mol = 0.01248 g

Answer: The mass of magnesium that reacted was 0.01248 g.