HYDRATES

Hydrates are compounds with a certain number of water molecules attached to them.  Their formulas look the same except that there is a “. # H2O” after it.                                  Example:  MgSO4 . 7 H2O

When determining the empirical formula for a hydrate, generally you will be determining the number in front of the H2O in the formula.  In order to determine this number, you will need to find the “mole ratio” between the moles of the compound and moles of water.

EXAMPLE:

A hydrated sample of sodium carbonate has a mass of 29.00 grams.  The sample is then heated and all water is removed.  The anhydrous salt that remains has a mass of 10.75 grams.  What is the empirical formula for the hydrated sodium carbonate?

STEP 1:  Determine the formula for sodium carbonate.  Na+1  CO3-2 à         Na2CO3

STEP 2:  Find moles of sodium carbonate.

10.75 g Na2CO3 | 1 mole Na2CO3 = 0.1014 moles Na2CO3    Na: 2 x 23.0 = 46.0

| 106 g Na2CO3                                                    C:   1 x 12.0 = 12.0

O:   3 x 16.0 = 48.0 +
106

STEP 3:  Find moles of water.

First, find grams of water…         hydrated sample = 29.00 grams

Anhydrous sample = 10.75 grams

Mass of water = 18.25 grams

18.25 g H2O | 1 mole H2O = 1.014 moles H2O

| 18 g H2O

STEP 4:  Find mole ratio of water to sodium carbonate.

1.014 moles   = 10                                 So  empirical formula for this hydrate is Na2CO3 . 10 H2O

0.1014 moles