 So here are two problems. If you're feeling confident pause the video now and try to solve them for yourself before I go through them. Okay, let's try the first one. So we have nitrogen gas reacts with hydrogen gas to produce ammonia gas. How many moles of hydrogen would be required to completely react with 3.2 moles of nitrogen? So the first step is to write out and balance the equation. Without the stoichiometric coefficients from the balanced equation you can't do any stoichiometry. This also means you need to take care to get your chemical formulae right because it's impossible to balance the equation if the formulae are wrong. So here we have nitrogen and hydrogen both diatomic, giving ammonia in H3 and we can balance it like this. Next I'm going to highlight the information that I'm given in the question. What we know is that there are 3.2 moles of nitrogen and what we want to work out is how many moles of hydrogen are needed to react with it. So to work this out we need to look at the ratio from the equation. We can see that for every one mole of nitrogen we need three moles of hydrogen. So the mole ratio is 1 to 3. So however much nitrogen we have we need three times as much hydrogen. Here we have 3.2 moles of nitrogen. So we multiply that by 3 to give 9.6 moles of hydrogen and that answers the question. 9.6 moles of hydrogen will react exactly with 3.2 moles of ammonia. OK, next problem. We're still using the same chemical reaction so no need to write out the reaction equation again. So what do we know? We're told that in a separate experiment, so we're starting from scratch but with the same chemicals, 4.5 moles of ammonia were produced and we want to know how much nitrogen and hydrogen must have reacted. This looks like we're working backwards but because it's all about ratios it doesn't actually matter whether we're using reactants to find products or vice versa. So we look at the mole ratios again from the equation. It's 1 mole of nitrogen is to 3 moles of hydrogen is to 2 moles of ammonia and the actual amount of ammonia that we have is 4.5 moles. So let's deal with the nitrogen first. The ratio is one part nitrogen to two parts ammonia. The 4.5 moles of ammonia that we actually have represents two parts. That's the two in the ratio. So to find out what one part is we just divide 4.5 by 2 and that tells us that 2.25 moles of nitrogen were needed. Now for the hydrogen. The ratio here is slightly more complex, 3 hydrogen to 2 ammonia. But you can see that if the 4.5 moles of ammonia represents two parts then we can divide by 2 to find out what one part is and then multiply by 3 to find the three parts of hydrogen and that tells us that 6.75 moles of hydrogen were needed. As a double check you can see that there should be three times as much hydrogen as nitrogen because the ratio of hydrogen to nitrogen is 3 to 1 and 2.25 times 3 does equal 6.75 moles so we're good. Now let me highlight these last two points on my list of tips here. Always annotate your calculations. By this I mean write explicitly what it is that you're calculating in each step. In simple problems like this it won't seem like it makes much difference but as your stoichiometry calculations become longer and more complex it becomes easier to make silly mistakes by accidentally mixing up values. Also don't forget your units. Alright let's try one last problem. Again try pausing the video to give this a go before you watch me working it out. We're going to look at the reaction in which iron reacts with oxygen to give iron 3 oxide which you're familiar with as rust. So first let's write and balance the equation. And now what do we know? We're told that we have 0.083 moles of iron and what we're calculating is how many moles of oxygen are needed to react with this amount of iron and how many moles of iron oxide are produced. So let's write out the mole ratio. We've got 4 iron to 3 oxygen to 2 iron oxide and we know the amount of iron present which is 0.083 moles. According to the ratio this represents 4 parts and we need 3 parts of oxygen to produce 2 parts of rust. So to find the amount of oxygen that's required we're going to divide 0.083 by 4 this will tell us what one part is and then multiply by 3 which gives us 0.06225 moles of oxygen. And we should think about significant figures. The stoichiometric coefficients are exact numbers so they effectively have infinite sig figs but we started with 0.083 moles of iron which has only 2 sig figs so our final answer should be rounded to 0.062 moles of oxygen. To find the number of moles of rust we take the moles of iron and again divide by 4 but now we multiply by 2 for the 2 parts of iron oxide and this tells us that 0.0415 moles of iron 3 oxide are produced or 0.042 moles when we round to 2 sig figs. Note that if you looked carefully at that last ratio 4 to 2 you'd see that you could simplify it to 2 to 1 and that would allow you to simply divide by 2 rather than dividing by 4 and multiplying by 2. Keep an eye out for shortcuts like this that could make your calculations faster.