Question Video: Calculating the Percentage Composition of a Theoretical Binary Compound Chemistry • First Year of Secondary School

Video Transcript

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

Percentage composition is a list of mass percentages for each element in the compound. So we’re looking for an answer with this kind of format where we have the mass percentage of 𝑋 and the mass percentage of 𝑌. We start off knowing that we have a sample mass of 12.5 grams and knowing that the only elements in our compound are 𝑋 and 𝑌. Thankfully, we’ve also been told that the mass of 𝑋 in the sample is 6.50 grams. But we don’t know the mass of 𝑌. However, that won’t stop us calculating the mass percentage of 𝑋 which is the mass of element 𝑋 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 don’t know the mass percentage for 𝑌. 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 𝑋 to get the mass percentage of 𝑌. Or we can work out the mass of 𝑌 by taking away the mass of 𝑋 from the mass of the sample and then dividing by the mass of the sample and multiplying by 100 percent. For this question, it isn’t actually necessary to work out the mass of 𝑌. So I’ve skipped it. Instead, we’ve worked out the mass percentage of 𝑌 by taking away the mass percentage of 𝑋 from 100 percent.

However, just in case you’re 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 𝑋 and 𝑌 having a sample mass of 12.5 grams will have a percentage composition of 52 percent 𝑋 and 48 percent 𝑌 if 6.50 grams of the sample is 𝑋.