Formula of a Hydrate Lab


Crucibles are VERY HOT; always handle them with tongs. DO NOT put hot crucibles on a balance! They cost $9.00 if you break it. Crucibles are VERY FRAGILE. Never carry them around without a heat-proof pad under it. Wear goggles at all times.

A hydrate is a chemical that has water molecules loosely bonded to it. The water molecules are not actually part of the formula, so the formula is written slightly differently. An example would be CaSO4 . 3H2O. This chemical would be called calcium sulfate trihydrate. When finding the mass of this chemical, you find the mass of the calcium sulfate and then add 3 times the mass of water to it. (40.08 + 32.066 + 4(15.999) + 3(2(1.0079) + 15.999)) = 190.19 g/mol.


The water can easily be removed from a hydrate just by heating strongly. You will be weighing a hydrate and heating it to remove the water (now called "anhydrous salt") and weigh it again. You can now find the percent of the anhydrous salt and the water. By finding a mol ratio, you can find out how many moles of water there are per mol of anhydrous salt. This number goes just before the H2O in the formula.

You will be using the hydrate CuSO4 . ?H2O.


Sample Calculation-
An empty crucible has a mass of 12.770 grams. The crucible and hydrate have a mass of 13.454 grams. After heating, the crucible and anhydrous salt have a mass of 13.010 grams. What is the formula of this hydrate of

MgSO4 . ?H2O?

Mass of hydrate = 13.454 - 12.770 = .684 grams

Mass of anhydrous salt = 13.010 - 12.770 = .240 grams

Mass of water = 13.454 - 13.010 = .444 grams


Moles of anhydrous salt = .240 grams MgSO4 x 1 mol MgSO4 = .00199 moles MgSO4

1 120.367 g MgSO4

Moles of water = .444 grams H2O x 1 mol H2O = .0246 moles H2O

1 18.0148 g H2O


Ratio of moles of water to moles of anhydrous salt = .0246/.00199 = 12

Therefore the formula is MgSO4 . 12H2O


Pre-lab questions:

1. What is the formula for copper (II) sulfate?

2. What is the mass of copper (II) sulfate?

3. Use the following data to find the formula of the hydrate BeO . ?H2O

Mass of hydrate = 8.61 grams

Mass of water = 3.60 grams

What is the mass of the anhydrous salt? What is the formula of the hydrate?



 1. Clean and dry a crucible and find its mass on an accurate balance.


2. Obtain a scoop of the hydrate from your teacher and find the mass again.


3. Heat the crucible under moderate heat for 3-5 minutes. Let the crucible cool and find its mass again.


4. Heat the crucible again under moderate heat for another 2 minutes. Place an inverted beaker over it while cooling. Find its mass again.


5. Repeat this heating and weighing until the mass doesn't change any more (stays within .05 grams).


6. Record all of the masses until there is no more water left.


7. Scrape your anhydrous salt into the disposal container and clean up your area.



 Mass of empty crucible: ____________ g

Mass of crucible and hydrate ____________ g

Mass of crucible and anhydrous salt trial 1 ____________ trial 2 ______________ trial 3 _______________

Mass of water _____________

Mass of anhydrous salt _____________


Post-Lab Questions:

 1. Why did we place a beaker over the anhydrous salt as it cooled?

2. If the bunsen burner left some black soot on the bottom of your crucible, how would this change your answer?

3. What is the formula of the hydrate? Show all calculations in the calculations section.


Questions? Comments??
Mike Horton