Question

A cylinder containers 27.5 L of oxygen gas at a pressure of 2.0 atm and a temperature of 320K. How much gas (in moles) is in the cylinder?

Answer 1

A bit over `"2 moles"`.

Explanation:

We use the Ideal Gas Law:

`n=(PV)/(RT)` `=` `(2.0*atmxx27.5*L)/(0.0821*L*atm*K^-1*mol^-1xx320*K)` `=` `??" moles"`

The choice of gas constant (`R`) is always problematic in theses sorts of problems. Chemists usually deal with atmospheres and litres. In other problems, sometimes you deal with pascals, and `""^@C`, and even (in the States) cubic feet and cubic inches, you may have to use different `R` values. Because, `atm` and `L` are sensible units, most of the time we use `R=0.0821*L*atm*K^-1*mol^-1`.

If you go thru and cancel the units in the expression above, you'll find that we get an answer in `"moles"`. This is a good thing because we wanted an answer in `"moles"`, so dimensional analysis is an extra way to check our calculation.