AQA Chemical Calculations (C2 - Higher)

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Chris Smith ­ Year 11 ­ Bourne Grammar School
Chemical Calculation Revision Sheet (assuming you can workout
relative mass and know what Ar and Mr are)
Empirical formula:
Q: Suppose 3.2g of sulphur reacts with oxygen to produce 6.4g of sulphur oxide.
What is the formula of the oxide?
step action S O
1 find masses 3.2 3.2
2 look up given Ar values 32 16
3 divide masses by Ar 0.1 0.2
4 find the ratio 1 2
Use the fact that the Ar of sulphur is 32 and the Ar of oxygen is 16.
Result= SO2
Atom Economy:
(Mass of desired products / mass of all) x 100
Percentage yield:
(Actual mass obtained/ Maximum obtainable mass) x 100
Mole (mol)
Mass (in g) / Mr (of element or compound).
Moles in a solution
Volume in litres x moles per litre of solution.
E.g. How many moles in 185cm3 of a 2mol solution = 0.185 x 2 = 0.37 mol
Percentage Mass
(Ar x No. of atoms (of the element) / Mr (of whole compound)) x 100
E.g. find percentage mass of sodium in sodium carbonate (Na2CO3)?
Ar of sodium= 23, Ar of carbon=12, Ar of oxygen=16
Mr of Na2CO3= (2x23) + 12 + (3x16) = 106
So if we substitute these into the equation it gives us:
(23x2)/ 106 all times by 100 = 43.4%.

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