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Percent Water in a Hydrate

Calculating Percent Water in a Hydrate

Calculate the percent of water in sodium carbonate decahydrate.
Description of Action
1. Write the formula for the given compound. Na2CO3 •10H2O
2. Calculate the molar mass of each part of the compound separately. Also, the number in front of the water molecule must be distributed and multiplied by the subscript of both the hydrogen and the oxygen in the water molecule. (Alternative: Water has a gram formula mass of 18.0 g/mol. Rather than go through the aforementioned process, multiply the coefficient in front of the water molecule by 18.0. In this problem there are 10 water molecules. 10 x 18.0 = 180.0, the same result as on the right.) Na: 2 x 23.0 = 46.0
C: 1 x 12.0 =   12.0
O: 3 x 16.0 =   48.0

H: 20 x 1.0 =    20.0
O: 10 x 16.0 = 160.0

3. Add the totals for each part together to find the molecular formula mass. 106.0 + 180.0 = 286.0
4. Divide each part’s mass by the molecular formula mass. Na2CO3: 106.0 ÷ 286.0 = 0.371
10H2O: 180.0 ÷ 286.0 = 0.629

5. Multiply each result by 100 in order to get a percentage. Na2CO3: 0.371 x 100 = 37.1%
10H2O: 0.629 x 100 = 62.9%

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