$\rm 20\,cm^3$ of ethyne, $\ce{C2H2}$, is reacted with $\rm 500 \,cm^3$ of oxygen. The equation for the reaction is:

$$\ce{2C2H2(g) + 5O2(g) → 4CO2(g) + 2H2O(l)}$$

What is the total volume of gas remaining at the end of the reaction?

(all volumes are measured at room temperature and pressure)

My effort: I think the answer should be $510\,\mathrm{cm^3}$ because the volume of the leftover oxygen is $450\,\mathrm{cm^3}$ and the volume of the products is $60\,\mathrm{cm^3}$ but the answer is $490\,\mathrm{cm^3}$. Can someone please explain why?

  • 1
    $\begingroup$ 490 cc seems to be the correct answer. Please show your work, and I would be happy to help you find your mistake. $\endgroup$ – getafix Sep 19 '15 at 5:29
  • $\begingroup$ Because they count water as liquid, that's why. $\endgroup$ – Ivan Neretin Sep 19 '15 at 6:04
  • $\begingroup$ I think you failed to notice that one of your products, i.e water is a liquid, and thus will not contribute to the total gas volume at the end of the reaction. $\endgroup$ – getafix Sep 19 '15 at 8:25

Here's a breakdown: First, you identify that ethyne is the limiting reagent and would be consumed completely. Second, you correctly calculated the volume of oxygen left that is $450\ \mathrm{cm^3}$. $20\ \mathrm{cm^3}$ of ethyne will completely react to produce $40\ \mathrm{cm^3}$ of carbon dioxide.

Thus, the total gas volume when the reaction reaches completion is $490\ \mathrm{cm^3}$.

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