Chemical amounts are measured in a unit called the mole, which has the symbol mol. One mole of a substance always contains the same number of particles, whether they are atoms, molecules, ions or electrons. This number is the Avogadro constant, which is 6.02 × 1023 per mole.
So one mole of carbon atoms (C) contains the same number of particles as one mole of carbon dioxide molecules (CO2). The particles are different, but there are the same number of them.
The mass of one mole of a substance in grams is numerically equal to its relative formula mass (Mr). Carbon dioxide has an Mr of 44, so one mole of it has a mass of 44 g.
To change between mass and moles use: number of moles = mass in grams ÷ relative formula mass. Rearranged, mass = number of moles × relative formula mass. For example, water has an Mr of 18, so 9 g of water is 9 ÷ 18 = 0.5 mol.