Video: Calculating Purity for a Chemical with All Numbers Provided

An impure sample of magnesium chloride has a mass of 50 g. After perfect purification, 45 g of magnesium chloride is recovered. What is the percentage purity of the original sample?

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Video Transcript

An impure sample of magnesium chloride has a mass of 50 grams. After perfect purification, 45 grams of magnesium chloride is recovered. What is the percentage purity of the original sample?

Magnesium chloride is a salt with formula MgCl2. And we’re told we have a sample containing magnesium chloride with a mass of 50 grams. But it’s impure. Some of the sample is magnesium chloride, but some of the sample is not. Next, we’re told this sample’s undergone perfect purification. When we perform a purification, our aim is to remove impurities. In a perfect purification, we’re removing all the impurities and not losing any of our target chemical. In this case, what we’re getting out is 45 grams of 100 percent pure magnesium chloride.

The question is asking us, what is the percentage purity of the starting sample? So, just to recap, we have our starting sample, which weighed 50 grams. It was then purified, removing the impurities, leaving 45 grams of magnesium chloride. So, we must’ve removed five grams of impurities because we can’t gain or lose mass in cases like this. We know there were five grams of impurities because 50 grams minus 45 grams is five grams.

Now, the question isn’t after the mass of impurities. It’s after the percentage purity of the original sample. And we calculate the percentage purity by taking the mass of the chemical in the sample, divide by the total mass of the sample, and multiply the result by 100 percent. The mass of chemical is the mass of magnesium chloride recovered. And the mass of the original sample was 50 grams. We can then multiply that by 100 percent. This gets us 0.9 times 100 percent, which is 90 percent. So, the mass of the original sample that was due to magnesium chloride is 90 percent.

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