### 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.