### Video Transcript

A mixture of nitrogen and oxygen
gas has a mass of 1.20 kilograms. After 250 grams of oxygen is
reacted with a metal, pure nitrogen is left behind. To two significant figures, what is
the percentage purity of nitrogen in the original mixture?

Nitrogen and oxygen, in this
question, refer to N₂ gas and O₂ gas, respectively. The question begins with a total of
1.20 kilograms of nitrogen and oxygen. This mixture is reacted with a
metal which removes all the oxygen, leaving only nitrogen. The question tells us that 250
grams worth oxygen was removed from the mixture. To work out the mass of what’s
left, we have to take 1.20 kilograms and convert it to grams before we take the 250
grams away. There are 1000 grams per
kilogram. So we can multiply 1.20 kilograms
by 1000 grams per kilogram to get 1200 grams.

Alternatively, you can recall that
kilo means 10 to the three or 1000. Either way, we end up at 1200
grams. To work out the mass of pure
nitrogen left behind, we take 250 grams away from 1200 grams, equalling 950
grams. The percentage purity of a
substance is equal to the mass of the pure substance, the mass of the substance on
its own, divided by the mass of the sample multiplied by 100 percent.

Generally, purity is measured by
mass not by moles. So we don’t need to do any
complicated molar calculations to work out the purity. We’ve worked out that the mass of
the pure substance nitrogen is 950 grams. And the mass of the sample in grams
is 1200.

Therefore, the percentage purity of
nitrogen in the original mixture is 950 divided by 1200 multiplied by 100
percent. This is equal to 79.1667
percent. But don’t forget, the question
asked us to give the answer to two significant figures. So our final answer for the
percentage purity of nitrogen in our 1.20 kilograms sample is 79 percent.