Percents By Weight
All men weigh 200 pounds. All women weigh 125 pounds. What is the percent by weight of woman in married couples? A married couple is one man and one woman. (No political implications intended.) The total weight is 325 pounds. The formula for percent is:
In this case the woman is the target, so the weight of the woman goes on top, and the total weight goes on the bottom of the fraction.
Notice that the units of pound cancel to make the percent a pure number of comparison.
The weights of atoms are the atomic weights. What is the percentage of chloride in potassium chloride? The atomic weight of potassium is 39.10 g/mol. The atomic weight of chlorine is 35.45 g/mol. So the formula weight of potassium chloride is 74.55 g/mol. The chloride is the target and the potassium chloride is the total. 35.45 g/mol / x 100% = 47.55198 % or 47.6 % to three significant figures.
You can do that with any part of a compound. What is the percentage of sulfate in beryllium sulfate tetrahydrate?
Notice that the examples here are done to two decimal points of the atomic weights. The problems in the practice bunch at the end of this chapter are done to one decimal point of the atomic weight.