Video: Calculating Oxidation State of Dihydrogen Phosphate Anion

What is the charge of the anion in Sr(H₂PO₄)₂?


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.

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