# Does pure water have a concentration?

Mar 29, 2017

There isn’t any.

#### Explanation:

By definition, “molarity” is the number of moles of a substance dissolved in a liter of solvent – which is usually water.

A pure solvent by itself does not meet the definition requirements of a “molar” solution. Theoretically you could have some molar concentration of water in a different solvent.

Mar 29, 2017

In fact we can speak of the $\text{molarity}$
or the $\text{concentration of water......}$

#### Explanation:

$\text{Molarity}$ $=$ $\text{Moles of solute (mol)"/"Volume of solution (L)}$

$= \frac{\frac{1000 \cdot g}{18.01 \cdot g \cdot m o {l}^{-} 1}}{1.00 \cdot L} = 55.2 \cdot m o l \cdot {L}^{-} 1$.

Water is concentrated stuff. With respect to moles per unit volume, it is a much more concentrated solvent than comparable systems, say $H F$, or $\text{acetic acid}$, and normally both of these materials are deployed as aqueous solutions. Given water's involatility, i.e. a boiling point of $100$ ""^@C under normal pressure, these 2 physical properties distinguish water as a REMARKABLE and UNUSUAL solvent.

What factors influence these properties?

Mar 29, 2017

If using molarity to express "number of moles of substance per litre volume of that substance" it is about 55.5 M.

#### Explanation:

Molarity is most commonly used to express the concentration of solutions, whereby it is the number of moles of solute per litre of solution.

However, somewhat confusingly it can also have a less common application for expressing the number of moles of a substance contained in a litre volume of that substance.

For water, the molar mass is approximately 18 g/mol, and a litre volume of water weighs 1000 g, so the number of moles of water in a litre of water is 1000 / 18 or about 55.5 moles per litre.