One mole of a substance is defined as the amount of that substance that contains the same number of particles (atoms, molecules, ions, or other entities) as there are in exactly 12 grams of carbon-12. In GCSE Chemistry, a mole is a unit of measurement used to express the amount of a substance in a sample. Work out the number of Cl 2 molecules in 6.34 mol. Worked example: Calculate the number of atoms in 450 g of chlorine (Cl2)ġ.Calculate the number of mole of chlorine in 450 g.Ģ. Number of moles = Mass/ Mr = given number of particles/ 6.02 x 10 23 We can use the equations to find the number of particles in any given mass of substance: Putting these two together, we can say that in one mole of the mass of any substance, there are 6.02 x 10 23 particles. One mole is the formula mass (Mr) of any substance in grams, and it is Avogadro’s number. We now have two ways of working out moles. It’s important to answer the question fully, so remember to state the units of grams when using moles. In 3 moles of sodium chloride, there are 3 moles of sodium. Here, we want to find the mass of sodium, so we can rearrange the formula we were using previously. Practice Question: What mass of sodium is present in 3 moles of sodium chloride? State the units in your answer. This leaves you will ‘moles x Mr’, which is the formula to work out the mass in this situation. Place your finger on top of the word ‘ mass’ in the triangle. To do this, we can use the triangle to help rearrange a formula. Sometimes, the exam question might ask you to find the mass of a substance when given the moles and relative formula mass. GCSE Chemistry – Moles
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