in chemistry the amount of product you get from a chemical reaction is known as the yield which we can measure in grams or moles to be more precise though we can use the terms actual yield to describe the amount we actually get when we carry out the reaction and theoretical yield to describe the yield we'd expect to get based on our calculations for example if we reacted 2 grams of hydrogen with 16 grams of oxygen then we should get 18 grams of water so that's our theoretical yield because 2 plus 16 equals 18. in practice though it might not make quite that much for example we could end up with only 15 grams or so and we'll call that our actual yield now there's a whole range of reasons why we might not make as much product as we expect but there are three particularly common ones that you need to know the first is that the reactants might not overreact so at the end some of the mixture will still be reactants this could happen if the reaction is particularly slow so it just hasn't had enough time to fully react or because it's a reversible reaction and it's reached equilibrium for example if we reacted nitrogen and hydrogen to make ammonia it wouldn't ever go to completion because it's a reversible reaction so some of the ammonia would keep breaking back down into nitrogen and hydrogen the second reason is that there could be side reactions which is where the reactants react to produce a different product than the one that we were expecting for example if we were looking at the same reaction as a minute ago some of the nitrogen might react with oxygen from the air to make nitrogen dioxide instead of going to make ammonia like we wanted it to and so we'd end up with less ammonia than we expected the third reason is more simple we could just lose some of the product during the process for example gaseous products can float off and escape or if we were filtering something we might not capture all of the liquid or solid to understand what we mean imagine that we were using a filter paper and funnel to filter out a solution and separate the liquid and solid if our aim was to capture all of the liquid it would be difficult because there'd be some liquid left in the beaker some left on the solid and also some left behind on the filter paper and so we're not actually getting all of the liquid into the test tube like we want similarly if we wanted to capture the solid we'd also struggle because we'd probably end up leaving some behind when we try to scrape it off the filter paper at the end the last thing we need to do is cover how to calculate the percentage yield which tells us what percentage of the theoretical yield we actually got to do this we take the actual yield and divide it by the theoretical yield and then we multiply it all by 100 to give us a percentage so it can range from zero percent if we don't get any product all the way to 100 if we got all the products that we predicted so if you think back to our reaction from the start of the video where we reacted to 2 grams of hydrogen with 16 grams of oxygen we predicted that our theoretical yield would be 18 grams but we said that our actual yield was only 15 grams so to find the percentage yield we do 15 divided by 18 and then times it by 100 which would give us 83.3 percent or in other words we got 83.3 percent of what we expected anyway that's everything for this video so hope you found that useful and it all made sense and hopefully we'll see you again soon