Chapter 15.2, Problem 14LC

Interpretation Introduction

Interpretation: The percent by mass of water in iron (II) sulfate heptahydrate ( $FeS{O}_4.7H_{2}O$ ) should be determined.

Concept Introduction: Mass percent of an element represents the contribution of that element in the given molecule with respect to other elements. Hydrate salts have water of crystallization bonded with the formula mass of the compound.

In the case of hydrate salt, it can be used to calculate the percent of the mass of water as:

  $\text{Mass percent = }\frac{\text{Mass of water in the hydrate salt }}{\text{molar mass of hydrate salt}}\times 100$

Answer to Problem 14LC

Percent of mass of water in $FeS{O}_4.7H_{2}O$ is $45.0\%$ .

Explanation of Solution

Molecular formula = $FeS{O}_4.7H_{2}O$

Molar mass of $FeS{O}_4.7H_{2}O$ = $277g/mol$

Molar mass of $FeS{O}_4$ = $152g/mol$

Mass of $H_{2}O$ = $277-152=125g$

  $\text{Mass percent = }\frac{\text{Mass of water in the hydrate salt }}{\text{molar mass of hydrate salt}}\times 100$

Substitute the values:

  $\begin{array}{l}\text{Mass percent of }{H}_{2}O\text{= }\frac{\text{Mass of water in the hydrate salt }}{\text{molar mass of hydrate salt}}\times 100\\ \text{Mass percent of }{H}_{2}O\text{= }\frac{\text{277-152g/mol }}{\text{277 g/mol }}\times 100\\ \text{Mass percent of }{H}_{2}O\text{ =}\frac{\text{125 g/mol }}{\text{277 g/mol }}\times 100\\ \text{Mass percent of }{H}_{2}O\text{ =}45.0\%\end{array}$

Conclusion

Hydrate salts have water of crystallization with the formula unit.