This question is about some of the elements in group 7 of the periodic table - Edexcel - GCSE Chemistry Combined Science - Question 5 - 2021 - Paper 1

To find the percentage by mass of chlorine in PCl₃, we first calculate the formula mass:

1. Formula mass of PCl₃:

   • P: 31.0 g/mol
   • Cl: 3 × 35.5 g/mol = 106.5 g/mol
   • Total = 31.0 + 106.5 = 137.5 g/mol

2. Percentage by mass of Cl:

   $\text{Percentage of Cl} = \frac{106.5}{137.5} \times 100 \approx 77.5\%$

Given 8.40 g of iron reacting, we first calculate the moles of iron:

1. Molar mass of Fe = 56.0 g/mol: $\text{Moles of Fe} = \frac{8.40}{56.0} = 0.15 \text{ moles}$

2. For Reaction A (1:1 ratio): 1 mole Fe reacts with 1 mole Cl₂. Thus, 0.15 moles of Fe will react with 0.15 moles of Cl₂. The mass of Cl₂ needed is:

   Molar mass of Cl₂ = 2 × 35.5 = 71.0 g/mol: $\text{Mass of Cl₂} = 0.15 \times 71.0 = 10.65g$

3. For Reaction B (2:3 ratio): 2 moles of Fe reacts with 3 moles Cl₂. Then, 0.15 moles of Fe would require: $\text{Moles of Cl₂} = 0.15 \times \frac{3}{2} = 0.225 \text{ moles}$

   Hence, the mass: $\text{Mass of Cl₂} = 0.225 \times 71.0 = 15.975g$

Since only 10.65g of Cl₂ is available, Reaction A is taking place.