Chemistry 11 : Percentage Yield and Percentage Purity

The formula for pecentage yield is

% yield = actual yield/theoretical yield * 100%

The actual yield is determined by experiment

The theoretical yield is a calculated value based on the amount(s) of reactant used in the reaction.

Why is the % yield not 100%?

Possible reasons Competing reaction – a reaction other than the desired one % occurring at the same time

The reason is less reactant reacts in the desired reaction lowing the yield

Experimental design or technique

The experimental technique or design lowers the yield because the reaction cannot go to completion or the product can’t be isolated completely or

Human error is poor experimental technique

Impure reactants – even new reactants have some impurities

MHbrO3 = (1.01 + 79.90 + 3 + 16.00)g/mol = 128.91g / mol MBr2 = 2*79.90g/mol = 159.80g/mol

Theoretical yield : mbr2 = 20.00g HBrO3 * 1/129.91g/mol * 3mol HBrO3 / 1mol HBrO3 * 159.80g/1mol

= 74.4g

47.3/74.4g * 100 = 63.6%’

From some reactions the percentage yield is given. From this you can calculate the actual yield

The purity of a substance is usually given using its percentage yield the percentage purity is given by the formula

% purity =