 Let's try doing some examples. Now in a second I'm going to ask you to pause the video and try this for yourself, but I will run through them also. Note that in order to do this you're going to have to be able to write down the formulae of each of these compounds or elements. So either have your notes ready from the previous videos or perhaps you're able to do them from your head. Okay so pause the video now and give it a go. Alright the first example hydrochloric acid reacts with magnesium to give magnesium chloride and hydrogen. So hydrochloric acid that's one of our common acids its formula is HCl. Magnesium is one of the elements, it's just Mg. Remember most of the elements can be written as single atoms. It's only those diatomic 7 where you have to worry about anything more complicated. And that becomes magnesium chloride and hydrogen. Let's just stick in the hydrogen first because it's simpler. That's one of our diatomic 7, H2. Now for magnesium chloride we need to work out its formula. It's an ionic compound because you can see it's made from a metal and a nonmetal. So we need to work out the ions. Magnesium is going to be Mg, it's in group 2 so it will have a charge of 2 plus. Chlorine is made, chloride is made of chlorine so it will be Cl, it's in group 7 so it's going to have a charge of 1 minus. When we do the crossover we get a formula of MgCl2 because 2 chlorides are needed to balance the 2 plus charge of the magnesium. Balancing the equation, let's do the hydrogen first. On the left we've got 1H and on the right we've got a hydrogen molecule which is made up of 2 hydrogen atoms. So we've got 1 on the left and 2 on the right. So let's put a 2 in front of our HCl. So we've now got 2 hydrogens on the left and 2 on the right. Remember you can't change the formula of hydrochloric acid, you can't make it H2Cl, it just doesn't work. Okay so now let's try the chlorides. We've just made this into 2 hydrochloric acids. Each hydrochloric acid has one chloride so that's 2 chlorides on the left. On the right the magnesium chloride also has 2 chlorides so that's also balanced. And now finally we'll check our magnesium. We have 1 on the left and we also have 1 on the right. So that's balanced. Okay.