Hi, I am Peter Edwards from Bluetutors. We teach children of all ages right from primary school to degree level and we find the highest quality teachers. Today I am going to teach you some maths. Okay ,so now we are going to talk about how to calculate the number of moles in a substance. So first, we should briefly explain what a mole is. A mole is a term just used to define a number of particles, it's a fixed number. It is 6.02 times 10^23 (ten to the power twenty three). This number is also known as Avogadro's constant, so it is just a number. It does not have any dimension, so when someone says “what is a mole?”, you can just say it's a number. However it's a useful way for us to work out the number of particles in a particular substance. For example, one mole of carbon is therefore six point zero two times twenty three carbon atoms weight of twelve grams, and similarly one mole of water is six point zero two times ten to the power twenty three water molecules, weight eighteen point zero two grams (18.02). So if we were to have let's say a hundred and twenty grams of carbon we would know that that was ten moles of carbon and therefore it would be ten times six point zero two times ten to the twenty-three. And well that is six point zero two times ten to twenty-four atoms and that's how you use moles and how you work out the number of particles in a particular substance. So if we were to do another example with water, I'll just get rid of that. I'll pick an easy example, so let's say we had one point eight zero two (1.802) kilograms of water. So what we would have to do is divide that by the weight of one mole of water. So one point eight zero two (1.802) divided by, I am going to write that number in kilos underneath to zero point zero one eight zero two (0.01802), and that is equal to one hundred. So we have a hundred moles of water in this - one point eight zero two kilos and a hundred moles of water would mean a hundred times six point zero two times ten to the power twenty three molecules of water, which is equal to six point zero two times ten to the power twenty five molecules of water. And that's how you work out the number of moles in a substance.

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