# How do you calculate the empirical formula of a hydrate?

Jan 10, 2015

Hydrates are compounds that contain water with a definite mass in the form of ${H}_{2} O$ in their molecular formula. These compounds often come in the form of a crystal which can then be heated in order to remove the water in the form of steam. An anhydrate is the substance that remains after the water from a hydrate has been removed through heating.

The steps you need to take in order to determine the molecular formula of a hydrate are

-calculate the mass of the water that has been removed;
-use this value to determine the moles of water that were initially present in the hydrate;
-use the mass of the anhydrate to determine the moles of anhydrate;
-determine the mole ratio between the water and the anhydrate;

Here's an example of how to do this: let's assume you want to determine the formula for the hydrate of barium chloride. You take a sample of the hydrate, weigh it, and find it has a mass of 4.13 g.

You then heat the sample in order to remove the water and find the weight of the anhydrate to be 3.52 g.

The first thing to do is to determine the moles of water that were in the sample

$\left(4.13 - 3.52\right)$ "g" * ("1 mole water")/("18.0 g") = 0.034 $\text{moles}$

You now go on to determine the moles of the anhydrous salt, $B a C {l}_{2}$

$3.52$ "g" * ("1 mole"BaCl_2)/("208.2 g") = 0.017 $\text{moles}$

The mole ratio between the water and the anhydrous salt is

$\left(\text{moles of water")/("moles of anhydrate}\right) = \frac{0.034}{0.017} = 2$

This means that for every mole of $B a C {l}_{2}$, you have 2 moles of water.

Therefore, the formula for the hydrate of barium chloride is $B a C {l}_{2} \cdot 2 {H}_{2} O$

Here are some other answers on how to go about determining the formula of a hydrate:

http://socratic.org/questions/how-do-you-determine-the-formula-of-a-hydrate?source=search

http://socratic.org/questions/how-can-i-determine-the-formula-of-a-hydrate?source=search

http://socratic.org/questions/how-do-you-determine-the-formula-of-a-hydrated-compound?source=search

Here is a video discussing how to find the empirical formula of copper sulfate.