### Video Transcript

A compound contains only the elements π
and π. A sample of the compound with a mass of
12.5 grams is found to contain 6.50 grams of π. What is the percentage composition of the
compound?

Percentage composition is a list of mass
percentages for each element in the compound. So weβre looking for an answer with this
kind of format where we have the mass percentage of π and the mass percentage of π. We start off knowing that we have a
sample mass of 12.5 grams and knowing that the only elements in our compound are π and
π. Thankfully, weβve also been told that the
mass of π in the sample is 6.50 grams. But we donβt know the mass of π. However, that wonβt stop us calculating
the mass percentage of π which is the mass of element π divided by the mass of sample
multiplied by 100 percent. This means we take 6.50 grams divided by
12.5 grams and then multiplied by 100 percent giving us 52.0 percent.

But we still donβt know the mass
percentage for π. We can get the mass percentage in one of
two ways. We know the mass percentages must sum to
100 percent. So we can take away the mass percentage
of π to get the mass percentage of π. Or we can work out the mass of π by
taking away the mass of π from the mass of the sample and then dividing by the mass of the
sample and multiplying by 100 percent. For this question, it isnβt actually
necessary to work out the mass of π. So Iβve skipped it. Instead, weβve worked out the mass
percentage of π by taking away the mass percentage of π from 100 percent.

However, just in case youβre interested,
it should be six grams. All that remains for our answer is to
write the percentage composition which is a list of the elements and their mass
percentages. A compound containing only the elements
π and π having a sample mass of 12.5 grams will have a percentage composition of 52
percent π and 48 percent π if 6.50 grams of the sample is π.