Question

What is the concentration of pure water at `298*K` given a `"density"` of `1.00*g*mL^-1`?

Answer 1

Just over 55 mol per litre.

Explanation:

Molarity is most commonly used as synonymous to "concentration", i.e. the number of moles of substance dissolved per litre of solution. However, it can also be used (albeit less commonly) to refer to the number of moles of pure substance per unit volume (of that substance).

In the case of pure water, we can assume density to be `1.0 g.cm^-3`, so 1000 ml of water will be 1000 g. Molar mass of water is 18.015 g/mol, so 1000 g contains 1000/18.015 = 55.509 mol per litre.