Unit 1: Topic 4 - Composition of Mixtures

Question 1

What is a mixture?

Answer 1

A mixture is a substance containing multiple compounds. There are molecules or formula units of
multiple types in a mixture, so you can’t express it with just one name or chemical formula.

For example, seawater is a mixture because it contains water, NaCl and other salts, and other
contaminants. Saying that seawater was just water or NaCl or anything else would be inaccurate since
it’s a mixture.

Question 2

What does it mean for a substance to be “pure”?

Answer 2

A pure substance is one that only contains one compound. Theoretically, every molecule or formula
unit in that substance has the same chemical formula. In practice, there will almost always be some
contamination, but substances that are almost all one compound can be treated as pure.

If someone says they have a sample of pure helium, that means that every atom in that gas sample is
He.

Question 3

What is percent purity?

Answer 3

Percent purity is a measure of how much of a given compound is in an impure sample. It is
calculated as a mass percent:

(mass of compound) / (total mass of impure sample) x 100% = % purity

Question 4

Example Problem:

What is the percent purity of a sample of MgCl2 that claims to be one mole but
actually weighs 100g? Assume that the sample does contain just one mole, and
the contaminant is the extra mass.

Answer 4

95.2%

The molar mass of MgCl2 is 95.205g/mol, so one mole should have a mass of 95.205g. Thus, the mass ratio of MgCl2 to the full sample is 95.205/100 = 95.2, and the percent purity is just that as a percent,
95.2%