Question

What is the total pressure (mmHg) of a gaseous mixture of 3.7 g of hydrogen gas and 9.1 g of oxygen gas in a 3.24-L flask if the partial pressure of hydrogen is 318 mmHg?

Answer 1

Approx. `1.5*atm`.

Explanation:

Given constant temperature, and volume, which we certainly have here (even though we don't know their magnitudes), pressure is proportional to the number of moles of gas, or vice versa.

`"Moles of dihydrogen"` `=` `(3.7*g)/(2.02*g*mol^-1)=1.83*mol`.

`"Moles of dioxygen"` `=` `(9.1*g)/(32.00*g*mol^-1)=4.97*mol`.

We are given that `P_"dihydrogen"=318*mm*Hg=(318*mm*Hg)/(760*mm*Hg*atm^-1)=0.418*atm`

And given the proportionality between moles and pressure,

`P_"dioxygen"=(4.97*mol)/(1.83*mol)xx0.418*atm=1.135*atm`.

And finally, `P_"Total"=P_"dihydrogen"+P_"dioxygen"`

`=` `(0.418+1.135)*atm`

Note that I converted the `mm*Hg` to units of `"atmospheres"`, because I really don't think `mm*Hg` should be used for pressures much above `1*atm, i.e. 760*mm*Hg`. There seems to be quite a few questions on these boards where mercury columns greater than `760*mm*Hg` have been used to measure pressure. Obviously, the person who set the question has never had to clean up a mercury spill.