Lab 4-C Formula of a Hydrate

Today, we had a lab on determining the percentage and moles of water in an unknown hydrate and writing the empirical formula of the hydrate. Basicly, all we did was to heat the unknown hydrate and weight the remaining hydrate to see how much water was given off by the hydrate. Also, we added a few drops of water to the remainings to see how the colour of hydrate have changed.

Here's the brief procedure of the lab.
1. Set up the equipments (crusible, pipestem triangle, iron ring, &　bunsen burner) as shown on the right. Heat the crusible to make sure that it is　dry. After cooling the crusible, determine & record the mass of the empty　crusible.
2. Place the hydrate into the crusible, determine & record the mass.
3. Heat the crusible with hydrate in it for about 5 min. Turn off the burner,let　it cool, and determine & record the mass of it.
4. Reheat the crusible with hydrate to make sure that all the water is driven　off.
5. Add a few drops of water to the comtents of the crusible and note any　changes occured.

A: Determining the percentages and moles of the water.

To determine the percentages of water, you need to know the mass of water given off by the hydrate. Simply, when you take the mass of the hydrate before heating and subtract the mass of the hydrate after heating (which is anhydrous salt ), that's the mass of water given off by the hydrate. Since you know the mass of hydrate and water, you can calculate the percent composition of the water. ( For calculating percent composition of a compound, see the previous post! )

% water = mass of water / mass of hydrate ｘ　100

Since you know the mass of water, it is easy to determine the moles of water present in hydrate! ( See the previous post for steps to calculate the moles of substance in a compound!)

number of moles of water = mass of water x 1 mole water / 18.0g water

B: Determining the empirical formula of the hydrate.

In order to determine the empirical formula, you need to know the moles of anhydrous salt present in hydrate. Take the same steps as part A to deter mine the number of moles of anhydrous salt.

Once you got the moles of water and anhydrous salt, you can determine the empirical formula drom finding the ratio of these two substances in the hydrate by making the number of moles into whole numbers. ( see the previous post!)





★These pictures are showing the possible colour change that can occur when heating a hydrate.

This video will explain more about hydrates and their formulas