Question

If the vapour pressure of a solution is `149.4*mm*Hg`, what is the vapour pressure in atmospheres?

Answer 1

At a given pressure, water (and any substance) has a characteristic vapour pressure. And here `mm*Hg` is used as a unit of pressure.

Explanation:

And at `100` `""^@C`, the characteristic vapour pressure of water is `1*atm`. That is at this temperature, the vapour pressure of the water is equal to the ambient pressure, and bubbles of water form directly in the liquid.....here the boiling point is specified. This is the relationship between boiling point and vapour pressure. and in fact we an reduce the boiling point by reducing the pressure.

At lower temperatures, the vapour pressure of the water is NOT one atmosphere, but it is still non-zero. It is convenient to quote the vapour pressure exerted by water in terms of `mm*Hg`, where `1*atm-="760 mm Hg"`...

In terms of atmospheres, we have a vapour pressure, often called the `"saturated vapour pressure"=("149.4 mm Hg")/("760 mm Hg"*atm^-1)=0.2*atm`.