### 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.