Formula Mass & the Mole Revision. formula mass the result of adding together the Relative Atomic Mass values of all of the atoms of all of the elements.

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Presentation transcript:

Formula Mass & the Mole Revision

formula mass the result of adding together the Relative Atomic Mass values of all of the atoms of all of the elements present in a chemical formula RAM values found in the data book page 7 RAM values are the relative masses of elements compared with the mass of carbon = eg H = 1, C = 12, O = 16 S = 32 & Cl = 35.5

SQA data book RAM values

Calculate the formula mass of water, H 2 O step 1 write the formula if it is not given 1 x O= 1 x 16= 16 2 x H= 2 x 1= 2 H2OH2O step 2 starting from the last element in the substance write down the number of atoms of each & its mass then add them all together to get the formula mass H2OH2O formula mass = 18

Calculate the formula mass of calcium nitrate, Ca(NO 3 ) 2 when there is a bracket in a formula everything in the bracket is multiplied by the number outside it 6 x O= 6 x 16= 96 2 x N= 2 x 14= 28 1 x Ca= 1 x 40= 40 formula mass = 140 Ca(NO 3 ) 2

the mole chemists use moles to describe how much of a pure substance they have as well as grams mol is short for mole 1 mol = the formula mass of a substance in grams (g) mass mol fm mass of substance in g formula mass moles Use this to do mol calculations

What is the mass of 1.5 mol of sodium chloride, NaCl ? using the mass (g) = mol x fmNaCl 1 x Cl = 1 x 35.5 = x Na = 1 x 23 = 23 fm = 58.5 mass (g) = 1.5 x 58.5 = 87.5 g

How many moles are in 0.32 g of methane, CH 4 ? CH 4 4 x H = 4 x 1 = 4 1 x C = 1 x 12 = 12 fm = 16 using the mol= mass fm mol= mass fm = = 0.02 mol