# Calculating Gas Pressure with Water Vapour

$\pu{4.0 L}$ of oxygen was collected by the displacement of water at $\pu{105.0 kPa}$ pressure and $\pu{30 ^\circ C}$. If the vapour pressure of water at $\pu{30 ^\circ C}$ is $\pu{4.2 kPa}$, calculate the pressure due to oxygen alone.

In answering this question, is it correct to just take $\pu{4.2 kPa}$ from $\pu{105.0 kPa}$? I feel as though I must be missing something, considering this was given to us under the title "Challenging questions on gases", but I can't see what other calculations would need to be done.

• Your analysis is correct. // Except do watch the significant figures. It is 105.0 kPa, not 105 kPa. – MaxW Nov 20 '17 at 7:25