3. What is a mole? Moles help to make large numbers more manageable. 1 mole of anything will contain 6x10 23 “bits”. So 1 mole of sodium will contain 6x10 23 atoms of sodium. 1 mole of oxygen will contain 6x10 23 molecules of oxygen (don’t forget oxygen is O 2 ), which means there are 12x10 23 atoms. You can think of moles like bags of potatoes. It can be time consuming to count a store room of potatoes but if they are in bags and every bag contains 100 potatoes it makes life much easier.
4. How heavy is a mole? The mass of 1 mole of a substance varies. It depends on its RAM or RMM. 1 mole of carbon has a mass of 12g but 1 mole of oxygen would have a mass of 32g. We can work out how many moles of a substance we have by dividing mass by RAM/RMM. Mass (g) RAM/RMM moles
5. Moles are useful because they help us work out how much of something we might make or need. E.g. what mass of CO is made if you burn carbon with 16g of O 2 ? Write an equation: 2C + O 2 2CO The equation tells us how many moles of everything there is: 2 moles of C + 1 mole of O 2 2 moles of CO Now work out how many moles you have of O 2 (it has to be O 2 because this is the only substance you are told anything about): Moles of O 2 = mass/RMM = 16/32 = 0.5 mol Now use the equation to work out how many moles of CO you would make: If there was 1 mole of O 2 there would be 2 moles of CO so moles of CO = moles O 2 x 2 = 1 mol Now convert the numbers of moles you have to mass: Mass of CO = moles x RMM = 1 x (12+16) = 28g
6. Moles and Gases Remember that 1 mole of ANY gas at room temperature and pressure is 24000cm 3 or 24dm 3 or 24l. Example: what volume of gas would be made if 6g of carbon reacted with oxygen? Write an equation: C + O 2 CO 2 The equation tells us how many moles of everything there is: 1 mole of C + 1 mole of O 2 1 mole of CO 2 Now work out how many moles you have of C (it has to be C because this is the only substance you are told anything about): Moles of C = mass/RMM = 6/12 = 0.5 mol Now use the equation to work out how many moles of CO you would make: If there was 1 mole of C there would be 1 mole of CO 2 so moles of CO 2 = moles C = 0.5 mol Now convert the numbers of moles you have to mass: Volume of CO 2 = moles x volume = 0.5 x 24 = 12dm 3
7. Moles and Concentration 1 mole of a substance dissolved in 1 dm 3 of water has a concentration of 1mol/dm 3 or 1M. Example: What is the concentration of a solution of NaCl containing 58.5g in 500cm 3 of water? Moles of NaCl = mass/RMM= 58.5/58.5 = 1mol Concentration = moles/vol = 1/0.5= 2M Moles Conc (mol/dm 3 ) Vol (dm 3 )
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