A very common quantitative problem in analytical chemistry is to determine how much of a substance is present in an unknown sample. Often, it may be difficult to measure the amount of the desired substance “directly,” so we try to determine the amount of some more conveniently measured material and relate the two amounts by reaction stoichiometry. Example: We seek to determine the purity of a sample of 0.285 g of CaCO3 (calcium carbonate) by considering the reaction with HCl (hydrochloric acid). We assume that any impurities do not react with the acid. The net balanced reaction (a must!) is: 1CaCO 3 + 2HCl -> 1CaCl 2 + H 2 O + CO 2 Suppose 17.6 mL of 0.250 M HCl is required to react completely with the sample. Remember, we are seeking to relate amt of one subst to another; to do so, we use the reaction stoichiometry (the recipe) and to use that, we need mols! Mols of HCl used:
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