I was given $4$ gases, which gases had the same temperature ,pressure and mass and was told to determine the gas with the greatest volume.

The gases were $\ce{CH4}$, $\ce{NO}$, $\ce{C2H2}$ and $\ce{CO2}$.

So I assumed the mass was $1$ gram and used the formula $n=\frac{\text{mass}}{\text{molar mass}}$ and found that $\ce{NO}$ had the greatest volume but the correct answer given to me was $\ce{CH4}$.

I cannot find out where I went wrong?

  • $\begingroup$ Check your molar masses and calculations. $\endgroup$ – jerepierre Jun 10 '17 at 13:43

First of all, if in a certain condition it is said that mass (or any other parameter) is constant, you cannot just assume ${1gm}$ (or any other number). Mass being constant means $\Delta m =0$, not that it is ${1gm}$. You have no way of knowing what the mass is (unless stated in the question, of course). The correct approach is to assume the mass to be $'m'\space{units}$. Now, this mass $'m'$ can assume any value, according to the condition.

Moving on, you have made a mistake while comparing molar mass and volume.

The ideal gas equation gives us the relation,

$$PV=\frac{\text{Mass}}{\text{Molar Mass}}RT$$

For your question,

$$V=\frac{1}{\text{Molar Mass}}.\frac{(\text{Mass})RT}{P}$$

$$V\propto \frac{1}{\text{Molar Mass}}$$

Recheck your calculations.

| improve this answer | |

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.