Question

The amount of Fe in a sample of 25 g of pyrrhotite (FeS) is $\underline{\hspace{1cm}}$ g. (Atomic wt. of Fe = 55.85 and S = 32.06) [round off to 2 decimal places]

Hint:

Always compute molar mass, then use the fraction of the element of interest to scale the given mass of compound.

Answer 1

Step 1: Calculate molar mass of FeS.
\[ M(\text{FeS}) = M(\text{Fe}) + M(\text{S}) = 55.85 + 32.06 = 87.91 \, \text{g/mol} \]

Step 2: Mass fraction of Fe in FeS.
\[ \text{Fraction of Fe} = \frac{55.85}{87.91} = 0.6353 \]

Step 3: Mass of Fe in 25 g sample.
\[ m_{\text{Fe}} = 25 \times 0.6353 = 15.883 \, \text{g} \]

Step 4: Round off to 2 decimal places.
\[ m_{\text{Fe}} = 15.88 \, \text{g} \]

Final Answer:
\[ \boxed{15.88 \, \text{g}} \]