Unit 1 Section 2 Chemical Yield

Theoretical Yield: The mass of product that should be formed in a chemical reaction.

1. Work out how many moles of the reactant you have.

2. Use the equation to work out how many moles of product you would expect that much reactant to make.

3. Calculate the mass of that many moles of product - and that's the theoretical yield.

E.g. 1.40g of iron filings react with ammonia and sulfuric acid to make hydrated ammonium iron(II)sulfate. Calculate the theoretical yield. Balanced equation is:

Fe(s) + 2NH3(aq) + 2H2SO4(aq) + 6H2O(l) >>> (NH4)2Fe(SO4)2.6H2O(s) + H2(g)

1. Work out how many moles of iron:

Moles of Fe = mass/molar mass = 1.40/55.8 = 0.025 moles

2. Work how many moles of product you would expect to make:

1 mole of Fe makes 1 mole of (NH4)2Fe(SO4)2.6H2O(s) so 0.025 moles of Fe = 0.025 moles of (NH4)2Fe(SO4)2.6H2O(s)

3. Calculate the mass of moles of product:

Theoretial yield = moles x molar mass = 0.025 x 392 = 9.8g

Percentage Yield: A comparison between the amount of product that should form during a reaction and the amount that actually forms in g. Can only be found if the theoretical and actual yield are known.

Percentage Yield (mass/g) = (Actual Yield / Theoretical Yield) x 100