Molarity of Pure Substances

Chemistry Tutorial

Molarity of Pure Substances

Intro

Molarity as well as the units that make up molarity are used indefinitely in chemistry, so fully understanding the concept is crucial. Some people do not realize that the idea of molarity extends past solutions and can be applied to pure substances (only the solvent). It is fun to get the science community thinking!

Sample Problem

Assuming the molar mass and density of water to be 18.01 g/mol and 1.00 g/ml, respectively, what is the molarity of pure water?

Solution

First keep in mind that there can only be a set number of mols in a given volume of space, and this number varies depending on the substance.

Assume: 1 L of water

Now how many mols of water are contained in that 1 L of water?

Recall: 1 L of water = 1000 ml water –> 1000 g water (via 1 g/ml density)

Solve for mols: (1000 g water)/(18.01 g/mol) –> 55.5 mols

Recall: we assumed 1 L water –> 55.5 mols/1 L –> 55.5 M



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