How many molecules are present in a sample of carbon dioxide, CO2 with a mass of 168.2?

Always remember that 1 mole is just a way of saying 6.02 x 10^23 molecules. Because this is a huge number it is easier to use moles. It corresponds nicely ecause 12 grams of carbon is one mole (note the Mr of carbon is 12) and 44g of CO2 is one mole of CO2 (note that CO2 has Mr 44). You need to use the equation n = m/Mr. In words this is: number of moles = the mass (in grams) divided by the Molar Mass. You can think of this as the number of moles is equal to the mass of the amount you have divided by the mass of one mole (the molar mass). To find the molar mass of CO2 you need to use the periodic table. 12 + 16 + 16 = 44 So you need to do: 168.2 divided by 44 which will give you number of moles.