Question Video: Relative Formula Mass of a Binary Compound with an Uneven Ratio of Elements | Nagwa Question Video: Relative Formula Mass of a Binary Compound with an Uneven Ratio of Elements | Nagwa

# Question Video: Relative Formula Mass of a Binary Compound with an Uneven Ratio of Elements Chemistry • First Year of Secondary School

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A substance has a formula unit AB₂. What is the relative formula mass of this substance in terms of Aᵣ(A) and Aᵣ(B), the relative atomic masses of A and B?

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### Video Transcript

A substance has a formula unit AB2. What is the relative formula mass of this substance in terms of Ar(A) and Ar(B), the relative atomic masses of A and B?

This one’s a little bit tricky because we haven’t been told what elements are involved. We’ve just got one A, whatever that is, and two of B. We’ve been asked for the relative formula mass of this substance, which means we need to work out the relative mass on a scale where an atom of carbon-12 would have a mass of 12.

To make things easier, let’s actually imagine a number line. We’d read an atom of carbon-12 as having a mass of 12. And what we’ve been given are some symbols for the relative atomic masses of A and B. I’m just going to imagine that the relative atomic mass of A is about four and that the relative atomic mass of B is about five. These numbers don’t actually matter. It’s just useful to draw it out.

To work out the relative formula mass, what we need to do is sum together the relative atomic masses of the constituents of the formula. With the formula AB2, we have one of A and two of B. Therefore, our relative formula mass, or RFM, is equal to one of the relative atomic mass of A plus two of the relative atomic mass of B.

We can imagine that on our diagram, where we’re getting gradually heavier as we move to the right. We have one of the relative atomic mass of A and two of the relative atomic mass of B. Since we’ll multiply the relative atomic mass of A by one, we can just remove the one and simplify our answer to the relative formula mass is equal to the relative atomic mass of A plus two times the relative atomic mass of B.

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