# Relative Formula Mass

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• Created by: Brooke
• Created on: 15-07-13 13:39
• Relative Formula Mass (RFM)
• Calculating RFM
• To calculate the RFM, all you have to do is add up the RAM of of all the atoms present in the formula
• Most candidates do not have a problem calculating RFM until they meet an example involving water of crystallisation
• The Mole
• The mole is a measure of the AMOUNT OF SUBSTANCE
• One mole (1 mol) is the amount of substance that contains 6x10 to the 23 particles (atoms, molecules or formulae) of the substance
• 6x10 to the 23 is known as AVOGADRO NUMBER
• For example, 1 mol of Sodium (Na) contains 6x10 to the 23 ATOMS of sodium
• Calculating the mass of one mole
• The mass of one mole of atoms is easily calculated. It is simply RAM expressed in grams
• The mass of one mole is also easily calculated by working out the formula mass and expressing it in grams
• Simple Calculations with moles
• 1. CALCULATING MASS FROM NO. OF MOLES
• Mass of substance (in grams) = amount of moles x RFM
• 2. CALCULATING AMOUNT FROM MASS
• Amount in moles = mass/RFM
• 3.CALCULATIONS WITH ATOMS OF ELEMENTS
• These types of calculations are important when working out the empirical formula of compounds
• Molar volume of a gas
• One mole of any gas has a volume of 24dm cubed (24000cm cubed) at room temp. This is called the molar volume of a gas
• 1. CALCULATING VOLUME OF GAS FROM AMOUNT OF GAS
• 2. CALCULATING AMOUNT OF GAS FROM THE VOLUME OF GAS