### Video Transcript

What is the mass of 0.443 moles of hydrazine, N2H4?

First, let’s remind ourselves of the relationship between mass and the number of moles. Here, I’ve displayed the relationship between mass, relative molecular mass, and number of moles in a triangular type shape, sometimes called a Sparky’s triangle. The way this works is whichever of the three values you want to calculate, you cover up — in this case, mass — and then perform the calculation of whatever’s left. In this case, it’s the relative molecular mass, 𝑀 r, times the number of moles.

In the question, we’re given the number of moles of hydrazine, 0.443. So, now, what we need to work out is the relative molecular mass. To do this, we need to add up the atomic mass of all the atoms in a molecule of hydrazine. This means we’re adding two lots of nitrogen and four lots of hydrogen. We can find the atomic mass of both nitrogen and hydrogen on the periodic table. You just have to find the correct element on the periodic table and look at the number below the element symbol.

So, for hydrogen, this is 1.00. For nitrogen, this value is 14.01. So, we can now substitute in these atomic masses into our initial equation. And this all works out as 32.02 grams per mole. Now, when we look back at our triangle on the right-hand side, we have to multiply this by the number of moles given in the question. So, the relative molecular mass is 32.02, and the number of moles is 0.443. This works out as 14.18486 grams. The value of the number of moles given in the question is to three significant figures. So, it would be sensible to give our answer also to three significant figures. So, the final answer is 14.2 grams.