Percentage yield

Groves1999

. A 3.56 g sample of calcium chloride was dissolved in water and reacted with an excess of sulfuric acid to form a precipitate of calcium sulfate. 2 The percentage yield of calcium sulfate was 83.4%. Calculate the mass of calcium sulfate formed. Give your answer to an appropriate number of significant figures. [3 marks]

Answer Moles of calcium chloride = 3.56 / 111.1 = 3.204 × 10–2 Moles of calcium sulfate = 3.204 × 10–2 × 83.4/100 = 2.672 × 10–2 Mass of calcium sulfate = 2.672 × 10–2 × 136.2 = 3.6398 = 3.64 (g)

I understand the first bit but don't understand why you divide the %yeikd by 100 and times if by moles

Reply 1

Kevin De Bruyne

21

Original post by Groves1999

. A 3.56 g sample of calcium chloride was dissolved in water and reacted with an excess of sulfuric acid to form a precipitate of calcium sulfate. 2 The percentage yield of calcium sulfate was 83.4%. Calculate the mass of calcium sulfate formed. Give your answer to an appropriate number of significant figures. [3 marks]

Answer Moles of calcium chloride = 3.56 / 111.1 = 3.204 × 10–2 Moles of calcium sulfate = 3.204 × 10–2 × 83.4/100 = 2.672 × 10–2 Mass of calcium sulfate = 2.672 × 10–2 × 136.2 = 3.6398 = 3.64 (g)

I understand the first bit but don't understand why you divide the %yeikd by 100 and times if by moles

Because in a perfect world with 100% yield, and with a 1:1 mol ratio, you would get an equal number of miles of the product as the original thing, however you only get a yield of 83.4% so you have to multiply the number of miles by that as the rest is not there.

For example, if For every £10 I put in I get £15 back, then my yield, which is the amount of extra money / original amount, would be 50% (5/10) * 100. My gain in this case is £5

So if I put in £20 then I would gain 0.5 * 20, which is £10.

I know I've introduced gain which is not the same as in this example but it's just to show you how multiplying by a ratio works.