Video: Calculating Purity after Given Mass of Contaminants

A mixture of nitrogen and oxygen gas has a mass of 1.20 kg. After 250 g of oxygen in the mixture is reacted with a metal, pure nitrogen is left behind. To 2 significant figures, what is the percentage purity of nitrogen in the original mixture?

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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.

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