The mole is a unit of measurement used in chemistry for the amount of a substance - OCR Gateway - GCSE Chemistry: Combined Science - Question 16 - 2021 - Paper 9

To calculate the number of moles of each substance, we can use the formula:

$\text{moles} = \frac{\text{mass (g)}}{\text{molar mass (g/mol)}}$

1. Moles of Iron (Fe):

   • Mass of Fe = 1.67 g
   • Molar mass of Fe = 55.8 g/mol
   • Moles of Fe = $\frac{1.67 \text{ g}}{55.8 \text{ g/mol}} \approx 0.03$ (to 2 decimal places)

2. Moles of Steam (H2O):

   • Mass of H2O = 0.72 g
   • Molar mass of H2O = $2 \times 1.0 + 16.0 = 18.0$ g/mol
   • Moles of H2O = $\frac{0.72 \text{ g}}{18.0 \text{ g/mol}} \approx 0.04$ (to 2 decimal places)

3. Moles of Hydrogen (H2):

   • Mass of H2 = 0.08 g
   • Molar mass of H2 = $2 \times 1.0 = 2.0$ g/mol
   • Moles of H2 = $\frac{0.08 \text{ g}}{2.0 \text{ g/mol}} \approx 0.04$ (to 2 decimal places)

From the reaction:

$\text{Fe}(s) + \text{H}_2\text{O}(g) \rightarrow \text{Fe}_x\text{O}_y(s) + \text{H}_2(g)$

Using the moles calculated above, we can balance the reaction. The coefficients give us the ratio of moles:

• 1 Fe reacts with 1 H2O to produce 1 Fe_xO_y and 1 H2.

Using the moles of reactants:

• 0.03 moles of Fe requires 0.04 moles of H2O and produces 0.04 moles of H2.

Thus, the balanced equation would suggest that 3 Fe will produce 4 H2O, leading to the formula for the oxide of iron as:

$\text{Fe}_3\text{O}_4$