Finding Percentage Yield and Purity

Definition of Yield?

Definition of Purity?

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Whenever an experiment is carried out, scientist like to calculate precisely how much product would be obtained. This is called the calculated yield. Also known as theoretical yield. For example,

Salicylic acid -> aspirin

How much of aspirin should we obtain from calculation if 100.0g of salicylic acid was used in this reaction?

Look at the chemical formula of salicylic acid, C7H6O3

So there are 7 C, 6 H and 3 O.

Hence the relative molecular mass is

7 x 12 + 6 x 1 + 3 x 16 = 138

Hence the number of mole of salicylic acid is 100g / 138 = 0.725 moles

So, 1 mole of salicylic acid will produce 1 mole of aspirin.

Hence 0.725 mole salicylic acid will produce 0.725 mole of aspirin.

Calculated mass of aspirin will be 0.725 mole X relative atomic mass of aspirin.

[NOTE: Relative atomic mass of aspirin is 180]

So, 0.725mole X 180 = 130.5g


Actual Yield

Since we will not obtain 130.5g, so the amount you would get from experiment is called actual yield.

From the question, actual yield is 121.2g

% yield = 121.2g / 130.5g X 100% = 92.9%