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# calaculating masses

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### calaculating masses

1. 1. MOLES <ul><li>At the end of this lesson I will be able to: </li></ul><ul><li>Calculate what mass of a substance is needed or is produced when different quantities of chemicals react </li></ul>
2. 2. Key Terms <ul><li>Relative atomic mass (RAM or A r ) </li></ul><ul><li>Relative formula mass (RFM) </li></ul><ul><li>Relative molecular mass (RMM or M r ) </li></ul><ul><li>Avogadro’s number </li></ul><ul><li>Mole </li></ul>Starter: Draw a quick mind map to show what these terms mean, pass to the person next to you to compare your mind maps. What have you said that is the same and what have you said that is different?
3. 3. 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
4. 4. 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
5. 5. What mass of iron is needed to make 16g of iron (III) oxide? Write an equation: 4Fe + 3O 2  2Fe 2 O 3 The equation tells us how many moles of everything there is: 4 moles of Fe + 3 moles of O 2  2 moles of Fe 2 O 3 Now work out how many moles you have of Fe 2 O 3 (it has to be Fe 2 O 3 because this is the only substance you are told anything about): Moles of Fe 2 O 3 = mass/RMM = 16/160 = 0.1 mol Now use the equation to work out how many moles of CO you would make: If there was 2 moles of Fe 2 O 3 there would be 4 moles of Fe so moles of Fe = moles Fe 2 O 3 x 2 = 0.2 mol Now convert the numbers of moles you have to mass: Mass of Fe = moles x RMM = 0.2 x 56 = 11.2g
6. 6. <ul><li>What mass of chlorine would be needed to make 73grams of hydrogen chloride? Equation: H 2 + Cl 2  2HCl </li></ul><ul><li>What mass of MgO would be produced by burning 12g of magnesium metal in excess oxygen? Equation: 2Mg + O 2  2MgO </li></ul><ul><li>Use the chemical equation to find the mass of carbon which reacts with 223g of lead oxide. Equation: 2PbO + C  2Pb + CO 2 </li></ul><ul><li>Extension </li></ul><ul><li>What mass of copper is formed when 8g of copper oxide is reduced with carbon? </li></ul><ul><li>How many grams of carbon dioxide could be obtained from heating 2.5g of calcium carbonate? </li></ul>Plenary
7. 7. Answers <ul><li>What mass of chlorine would be needed to make 73grams of hydrogen chloride? Equation: H 2 + Cl 2  2HCl </li></ul><ul><li>Moles of HCl=mass/RMM=73/36.5=2, therefore moles of Cl 2 =1. </li></ul><ul><li>Mass of Cl 2 =molesxRMM=1x 35.5x2= 71g </li></ul><ul><li>What mass of MgO would be produced by burning 12g of magnesium metal in excess oxygen? Equation: 2Mg + O 2  2MgO </li></ul><ul><li>Moles of Mg=mass/RMM=12/24=0.5, therefore moles of MgO=0.5 </li></ul><ul><li>Mass of MgO=molesxRMM=0.5x 40= 20g </li></ul><ul><li>Use the chemical equation to find the mass of carbon which reacts with 223g of lead oxide. Equation: 2PbO + C  2Pb + CO 2 </li></ul><ul><li>Moles of PbO=mass/RMM=223/223=1, therefore moles of C=0.5. </li></ul><ul><li>Mass of C=molesxRMM=0.5x 12= 6g </li></ul>
8. 8. Answers to Extension Questions <ul><li>What mass of copper is formed when 8g of copper oxide is reduced with carbon? </li></ul><ul><li>Equation: 2CuO + C  2Cu + CO 2 </li></ul><ul><li>Moles of CuO = mass/RMM= 8/80 = 0.1, therefore moles of Cu=0.1 </li></ul><ul><li>Mass of Cu = molesxRMM = 0.1x64= 6.4g </li></ul><ul><li>How many grams of carbon dioxide could be obtained from heating 2.5g of calcium carbonate? </li></ul><ul><li>Equation: CaCO 3  CaO + CO 2 </li></ul><ul><li>Moles of CaCO 3 = mass/RMM = 2.5/100 = 0.025, therefore moles of CO 2 = 0.025 </li></ul><ul><li>Mass of CO 2 = molesxRMM = 0.025 x 44 = 1.1g </li></ul>