Using percentages

Percentage by mass is commonly used in chemistry
The percentage by mass of component A can be calculated if the mass of A and the total mass are known.

%A =	m(A)	× 100
	m(total)

per cent means per hundred
To calculate this determine the mass fraction and then multiply by 100.

Some important points about percentages:

Because the unit is %, the unit of mass of both A and the total MUST be the same so that they divide into one another to give 1.

This means that that if the units are different, the unit (and the number) for one of these must be adjusted to make them the same.

Example: What is the percent by mass of table salt in a mixture if 150 g of the mixture contains 300 mg of table salt?

%NaCl =	300 mg	× 100
	150 000 mg

%NaCl =	0.3 g	× 100
	150 g

Usually it is most convenient to convert both to the larger unit so that the numbers are easier to understand.

Because of the way percentage is defined, if a sample contains only substances A, B and C, the sum of their mass percentages MUST be 100%.