Question

Equal mass of iron and sulphur are heated together to form FeS. What fraction of the original mass of excess reactant is left unreacted? (Fe = 56, S = 32)

• A. 0.22
• B. 0.43
• C. 0.86
• D. 0.57

Answer 1

The correct option is B 0.43

$\text{Let, initially, we take m}gm\text{. each of}Fe\text{and}{S}_8\text{.}$

$Fe+\frac{1}{8}{S}_8⟶\text{FeS}$

$t=0\frac{m}{56}\frac{m}{256}0$

$\begin{array}{cc}t=t-& (\frac{m}{256}-\frac{m}{56}\times \frac{1}{8})\\ & =\frac{3m}{1792}\end{array}$

$\begin{array}{c}\text{Here, Fe is limiting reactant while}{S}_8\text{is excess reactant.}\\ \text{so, moles of so left unreacted}=\frac{3m}{1792}\end{array}$

$\therefore \text{mass of}{S}_8\text{left unreacted}=\frac{3m}{1792}\times 256=\frac{3m}{7}$

$\text{So,}\frac{3}{7}=0.43\text{fraction of initial mass of}{S}_8\text{is left unreacted.}$