### Video Transcript

What is the charge of the anion in
Sr(H2PO4)2? One minus, two minus, two plus,
three minus, or one plus.

So this question is asking us for
the charge of the anion. And if you’re not familiar with
this particular anion, dihydrogen phosphate, this might seem like a very difficult
question. But we can work it out using our
knowledge and the periodic table. Firstly, we notice that the
compound we’re asked about is neutral overall. This means that the charges on the
cation and the anion must all add up to zero at the end. Let’s start by looking at the
strontium cation.

If we look at our periodic table,
we’ll see that strontium is in group two. This means that our strontium
cation is going to have a two plus charge. You may be more familiar with
something like calcium two plus or magnesium two plus, which are both in the same
group. So we worked out the positive
charge. Now we just need to work out the
negative charge. Because overall we want to end up
with a neutral charge, we need to have a two minus charge on the dihydrogen
phosphate anion. However, be careful! We actually have two equivalents of
the dihydrogen phosphate anion. So if two dihydrogen phosphate
anions have an overall charge of two minus, that must mean that a single dihydrogen
phosphate anion must have a charge of one minus.

So with a strontium cation of two
plus and two lots of the dihydrogen phosphate anion, each with a one minus charge,
giving us two minus overall, everything adds up to zero. So we have a neutral compound. So the correct answer to this
question is one minus.