Purity is also a form of measuring concentration (generally used for solids), it says how much of a certain sample of a compound is actually made from the compound you want. Suppose you have 50g of a NaCl sample, but it only has 45g of NaCl. The other 5g are called impurities, so only 90% (45g/50g) of the sample in mass is actually made of NaCl, this percentage is called the purity and its mathematical expression is
\(\text{purity} = \frac{\text{mass of the pure compound}}{\text{total mass of the sample}}\)
In the context of stoichiometry and chemical reactions in general, the concept of purity is used to more accurately assess how much of a product will be formed by some amount of reactants. Take the reaction for combustion of ethanol
\(\ce{C2H5OH + 3O2 -> 2CO2 + 3H2O}\)
On this reaction, 46 g of Ethanol (\(\ce{C2H5OH}\)) forms 88 g of \(\ce{CO2}\) (If you don’t quite understand why that happens, you probably will after checking the next page, reaction stoichiometry. However, if you have a 46 g sample of ethanol, you won’t have 88 g of \(\ce{CO2}\) after the reaction, because you also have to take into account the purity of your sample. If the purity of your ethanol sample is 75%, only 75% of 88 g of \(\ce{CO2}\) will actually be formed, which is 66 g of \(\ce{CO2}\).
Written by Jailson Godeiro