# How do you convert .75 atm to mm Hg?

May 21, 2016

$\text{1 atm}$ $\equiv$ $\text{760 mm Hg}$

#### Explanation:

So why can I make the above statement. Have I not equated units of length, millimetres, with units of pressure, force per unit area?

Well, of course I have, but why is it justifiable in this context?

It is a fact that a pressure of $1$ $a t m$ will support a column of mercury $0.760$ $m$ high. It does not matter how thick the column is, though as responsible chemists, who know that mercury is (i) expensive, and (ii) relatively hazardous, we want to make the column as thin as possible so as to use the least volume of mercury.

And this $0.75 \cdot a t m$ $=$ $0.75 \cdot \cancel{a t m} \times 760 \text{ mm Hg } \cancel{a t {m}^{-} 1}$ $\cong \text{ 580 mm Hg}$

Chemists routinely use mercury columns as they are a visual and convenient way to measure pressure, and it is something that could have been done 200 years ago. Of course, chemists run the risks of breaking the column and spilling the mercury, which is a major and very inconvenient cleanup job.