Percent Composition by formula

This is the theoretical value. This mean that this percentage was calculated using the formulas and not by an experiment. Therefore, this value is what we strive to get. Usually this will be higher than the experimental percentage because of human error or inaccuracy in measuring. The closer you get to the theoretical percentage (percent composition by formula) the better your experiment went.

To calculate the theoretical percentage or percent composition by formula you need to add up the masses for each atom represented in the formula. In this example you have H₂O and MgSO_4. As you can see from the figure it is partially complete. Complete it on your own .

			Mg = 24.3 g/mol H = 1.0 g/mol O = 16.0 g/mol S = 32.1 g/mol
		H = 1 H = 1 O = 16 + 18 7 x 18 =126
	Water

7 H₂0______ x 100 = __?_ _____________________________ x 100 = ____

H = 1

H = 1

O = 16 +

18

7 x 18 =126

Mg = 24.3

S = 32.1

O = (16 x3) = 64 +

120.4

MgSO₄ · 7H₂O

		+
Water and salt