# Calculations What you need to know: Relative formula mass Empirical formula % composition by mass Use balanced equations to calculate masses of reactants.

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Calculations What you need to know: Relative formula mass Empirical formula % composition by mass Use balanced equations to calculate masses of reactants and products Yields

Quick quiz – molecular mass and empirical formula **YOU MAY USE REVISION CARDS FROM PREVIOUS LESSONS** 1.Calculate the molecular mass (M r ) of: – Sodium sulfate – Calcium hydroxide – Magnesium nitrate 2.If 9g of aluminium reacts with 35.5g of chlorine, what is the empirical formula of the compound formed? 3.A compound contains 16g of sulfur and 24g of oxygen. What is its empirical formula? 4.A compound contains 44.8g of iron and 19.2g of oxygen. What is its empirical formula?

% composition by mass A typical McDonald’s meal consists of a burger (250g) chips (100g) and a drink (200g) What is the percentage composition of the meal? 1.Total mass = 250 + 100 + 200 = 550g 2.What % is burger? = (250/550) x 100 = 45.4% 3.What % is chips? = (100/550) x 100 = 18.2% 4.What % is drink? = (200/550) x 100 = 36.4%

percentage compositions of compounds: We use the same method to work out the percentage compositions of compounds: Example: What is the percentage composition of NaOH? Stage 1: find the relative atomic mass (RAM) numbers of the elements Na = 23 O = 16 H = 1 Stage 2: add these RAM numbers together to get the formula mass: 23 + 16 + 1 = 40 Stage 3: work out each elements percentage composition using their RAM numbers and the formula mass Na = 23/40 X 100 =57.5% O = 16/40 X 100 = 40% H = 1/40 X 100 = 2.5% Stage 4: Add up the individual percentages and check that the total equals 100%

Your turn……… 1.Calculate the percentage by mass of nitrogen in each of the following: a)Ammonia NH 3 b)Ammonium chloride NH 4 Cl c)Ammonium sulfate (NH 4 ) 2 SO 4 2.NOW MAKE A REVISION CARD

Calculating the masses of reactants and products Example: what is the mass of carbon dioxide formed when 36g of carbon is burnt? Step 1: write the balanced equation: C + O 2  CO 2 Step 2: work out relative masses of required chemicals: C = 12 CO 2 = 44 Step 3: divide both by smallest number to find ratio: C = 12/12 = 1 CO 2 = 44/12 = 3.67 (1g C produces 3.67g of CO 2 ) Step 4: find mass of CO 2 formed when 36g C burnt 1 x 36 = 36g 3.67 x 36 = 132g ANSWER: 132g OF CARBON DIOXIDE IS FORMED WHEN 36g OF CARBON IS BURNT!

Your turn…. 1.What mass of water is formed when 4g of hydrogen is burnt? 2.What mass of sodium chloride is formed when 10g of sodium hydroxide reacts with hydrochloric acid? 3.NOW MAKE A REVISION CARD

% YIELDS

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