# How can I find the mole ratio of anhydrous salt to water?

May 6, 2014

Sounds like you are working on lab analysis. A lot of teachers will use hydrated salts such as $C u S {O}_{4}$ or $M g S {O}_{4}$ to teach the concept of mole ratio.

#### Explanation:

What was the mass of the hydrated (wet) salt? (${m}_{1}$)?
What was the mass of the salt after it was dried by heating? (${m}_{2}$)?

${m}_{w}$ = mass of water

${m}_{w}$ = ${m}_{1}$ - ${m}_{2}$
convert this to moles of water by dividing by the molar mass of water which is 18.02 g/mol

Convert the mass of the anhydrous salt (${m}_{2}$) by dividing it by the molar mass of the anhydrous salt.

The moles of salt should be the smaller of the two values. Divide the moles of water by the moles of salt to find the ratio. This will allow you to find the mole ratio. Common ratios will range from 1:2 up to 1:10.

Here is an example lab & calculation using copper sulfate.

Here is a video which discusses that type of calculation.