Chapter 10.5, Problem 74PP

Interpretation Introduction

Interpretation:

The name and formula of hydrate according to the given composition needs to be determined.

Concept introduction:

Empirical formula is the simplest formula of any organic or inorganic compound that represents the simple ratio of all atoms present in the molecule. It can be calculated with the help of elemental composition of molecule. Certain steps must be used to get the empirical formula:

• Consider mass % as mass in grams and calculate moles of element with the help of molar mass.
• Calculate the moles of each element in least whole number.
• Write the number of each atom as subscript to write the empirical formula.

Answer to Problem 74PP

Formula = $\text{MgSO}\text{.}{\text{7H}}_{\text{2}}\text{O}$

Name = Magnesium heptahydrate or Epsom salt

Explanation of Solution

Given information:

3 $\text{MgSO}$ = 48.8 %

${\text{H}}_{\text{2}}\text{O}$ = 51.2 %

Molar mass of $\text{MgSO}$ = 120.3 g/mol

Molar mass of ${\text{H}}_{\text{2}}\text{O}$ = 18.0 g/mol

Calculate moles of each part:

$\begin{array}{l}\text{Moles of MgSO}\text{ = }\frac{\text{48}\text{.8 }}{\text{120}\text{.3 }}\text{= 0}\text{.4056}\\ {\text{Moles of H}}_{\text{2}}\text{O = }\frac{\text{51}\text{.2}}{\text{18}\text{.0 }}\text{= 2}\text{.844}\end{array}$

Divide by least moles to get the whole number:

$\begin{array}{l}\text{Moles of MgSO}\text{ = }\frac{\text{0}\text{.4056}}{\text{0}\text{.4056}}\text{= 1}\\ {\text{Moles of H}}_{\text{2}}\text{O = }\frac{\text{2}\text{.844}}{\text{0}\text{.4056 }}\text{= 7}\end{array}$

Thus the empirical formula must be $\text{MgSO}\text{.}{\text{7H}}_{\text{2}}\text{O}$

Conclusion

Formula = $\text{MgSO}\text{.}{\text{7H}}_{\text{2}}\text{O}$

Name = Magnesium heptahydrate or Epsom salt