Question #2c98c

1 Answer
Feb 21, 2015

The mole ratio between the water and the anhydrous salt is usually determined using the mass of the hydrate, the mass of the anhydrous salt, and the mass of the evaporated water.

Basically, you would prepare a sample of the hydrate and heat it until all the water has evaporated. Once that happens, the hydrate sample will be reduced to the andyrous salt, which means you can weigh it again and see how much water the sample contained.

Here's an example using nichel (II) sulfate. Assume you have a sample of NiSO4 hydrate that weighs 40.00 g. After heating the sample until the water is evaporated, its weight is now 20.74 g. Determine the formula of the hydrate.

The first thing you need to do is determine how much water was evaporated; this is done by subtracting the mass of the anhydrate from the mass of the hydrate

mwater=mhydratemanhydrate

mwater=40.00 g20.74 g=19.26 g

Next, determine the number of moles of water and anhydrous salt you have by using their respective molar masses

19.26 g water1 mole18.0 g=1.070 moles water, and

20.74 g anhydrate1 mole154.8 g=0.1340 moles anhydrate

Now divide both these numbers to find the mole ratio water has with the andhydrous salt

1.070 moles water0.1340 moles anhydrate=7.9858.00

This means that you get 8 moles of water for every mole of NiSO4; as a result, the formula will be

NiSO48H2O

One mole of NiSO4, 8 moles of H2O.