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Aluminium oxide can be reduced to produce aluminium: Al2O3
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2Al + 1 2 O2
The equation shows that one mole of aluminium oxide produces two moles of aluminium.
Calculate the mass of aluminium oxide needed to produce 540g of aluminium. 540 = 20mol amount of aluminium = 27 20 = 10mol amount of aluminium oxide required = 2 formula mass of aluminium oxide = (27 × 2) + (16 × 3) = 102
amount of aluminium = mass atomic mass Relative atomic masses (Ar): Al = 27 and O = 16. The equation shows that one mole of aluminium oxide is needed to produce two moles of aluminium, so divide by two.
mass of aluminium oxide needed = 10 × 102 = 1020g •
The numbers needed to balance an equation can be calculated from the masses of the reactants and the products using moles.
mass of aluminium oxide needed = amount (mol) × formula mass
In a chemical reaction, 72g of magnesium was reacted with exactly 48g of oxygen molecules to produce 120g of magnesium oxide. Use the number of moles of reactants and products to write a balanced equation for the reaction. 72 amount of Mg = = 3mol 24 48 amount of O2 = = 1.5mol 32 120 amount of MgO = = 3mol 40
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3Mg + 1.5O2
3MgO
2Mg + O2
2MgO
Use the masses of the reactants to calculate the number of moles present.
Divide the number of moles of each substance by the smallest number (1.5) to give the simplest whole number ratio. This shows that 2 moles of magnesium react with 1 mole of oxygen molecules to produce 2 moles of magnesium oxide.
Limiting Reactants Sometimes when two chemicals react together, one chemical is completely used up during the reaction. When one chemical is used up, it stops the reaction going any further. It is called the limiting reactant. The other chemical, which is not used up, is said to be in excess.
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Concentration of Solutions • •
Many chemical reactions involve solutions. The concentration of a solution may be given by the mass of solute per given volume of solution. For example, if 2.00g of solute was dissolved to form 1.00dm3 of solution, the concentration of the solution would be 2.00g/dm3.
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Quick Test 1.
69g of sodium reacts with chlorine to produce sodium chloride: 2Na + Cl2 2NaCl a) Calculate the number of moles of sodium present. b) Calculate the number of moles of chlorine (Cl2) that would be required to react exactly with the sodium. c) Calculate the mass of chlorine that would be required to react exactly with the sodium. HT
Key Words mole (mol) HT Avogadro constant HT limiting reactant concentration HT
Quantitative Chemistry: Revise
227841 AQA Higher Tier Combined Science All-in-One_P088_105.indd 105
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6/29/16 11:17 AM