Question

A sample of dolomitic limestone containing only `CaCO_3` and `MgCO_3` was analyzed. When a 0.2800 gram sample of this limestone was decomposed by heating, 75.0 mL of `CO_2` at 750 mmHG and 20°C were evolved. How many grams of `CO_2` were produced?

Answer 1

`MCO_3(s) + Delta rarr MO(s) + CO_2(g)uarr`

Approx. `140 *mg` gas are evolved.

Explanation:

We use the Ideal Gas equation to access the moles of carbon dioxide evolved:

`n=(PV)/(RT)` `=` `((750*mm*Hg)/(760*mm*Hg*atm^-1)xx75.0xx10^-3*L)/(0.0821*L*atm*K^-1*mol^-1xx293*K)`

`=3.1xx10^-3mol`.

`"Mass of carbon dioxide produced"=3.1xx10^-3molxx44.0*g*mol^-1=??g`

The key to doing this problem was (i) knowing that `760*mm*Hg-=1*atm`; and (ii) using the right gas constant, `R`. I have always liked `R=0.0821*L*atm*K^-1*mol^-1` because it uses units that chemists would habitually use. The other gas constants tend to use whack, non-intuitive units.