Video Transcript
The mass of a substance, lowercase 𝑚; its molar mass, capital 𝑀; and the number of moles, lowercase 𝑛, can be related by a single formula. What is that formula?
The molar mass tells us the mass of one mole of a substance. We can determine the molar mass of a substance by looking at the atomic mass in the periodic table. As carbon has an atomic mass of about 12 atomic mass units, the molar mass of carbon is about 12 grams per mole. This means that if we’re to measure out 12 grams of carbon, we would have one mole of carbon atoms, or about 6.022 times 10 to the 23 carbon atoms, as that is how many things are in one mole of a substance.
Knowing the number of moles present in a sample is very important in chemistry. But it would be quite difficult to count the number of atoms present in a sample. It’s much easier to measure the mass of a substance. The molar mass is very useful because it helps us relate the mass to the number of moles. If we want to know the number of moles present in a sample and we know the substance’s mass, we simply need to divide the mass by the molar mass. For example, we could calculate the number of moles present in this sample of carbon. All we need to do is divide the mass by the molar mass. The grams cancel, and we’d find that there was one mole of carbon atoms in the sample.
So the formula that relates the mass of a substance, the molar mass, and the number of moles is 𝑛 equals lowercase 𝑚 divided by capital 𝑀.
