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Question for percent yield

Submitted by Student007 on Mon, 10/19/2009 - 03:23

What might be the cause of a percent yield higher than the maximum?

There could be many reasons eg impurities still present in the product eg when precipitating a compound, some particles of reactant could still be in the filtered solid and remain there if dried before sufficient washing

In a decomposition, eg CaCO3 ----> CaO  +CO2. If CaO is collected and weighed it will appear to be heavier if the decomposition is incomplete and it still contains some CaCO3. So the actual yield is greater than the percentage yield.

I remember an experiment where students determined the purity of marble chips and this sometimes gave greater than 100% yield. I wondered if the presence of MgCO3 impurity would cause this problem. For example, if CaCO3 = 100g/mol and MgCO3 = 84g/mol, then 100g CaCO3 would react with 2 mol HCl.

However if 100g contained 50g CaCO3 (0.5 mole) and 50g MgCO3 (>0.5 mole) then this would react with more than 2 mole of HCl and make you think there was more than 100 g of CaCO3 in the sample (Obviously I exaggerated the impurity here)

Hope this gives you some help.