I have a quick question in Stoich and Gasses, I however could not get any far, any answer will be appreciated. Thank You.

A sample of $\ce{Pb(NO3)2}$ is heated, yielding $\ce{O2, NO2\ gas,\ and\ solid\ PbO}$. A $\ce{293 \ ml}$ gas sample, measured at $\ce{200C}$ and $\ce{1\ atm}$, is collected. What was the mass of the $\ce{Pb(NO3)2}$ sample?

I feel like in this situation I need to have only one gas so I can apply the Ideal Gas Law to solve and find the mass of $\ce{Pb(NO3)2}$. What should I do there are two gasses and I don't know the volumes or the masses?

$$\ce{2Pb(NO3)2 -> O2 + 4NO2 + 4PbO}$$

  • $\begingroup$ You can still assume ideal gas conditions even if it's a mixture of several gasses. You also need a reaction equation. $\endgroup$ – tschoppi Dec 8 '14 at 23:31
  • $\begingroup$ @tschoppi I don't quite grasp that. $\endgroup$ – Asker123 Dec 9 '14 at 0:28
  • $\begingroup$ You don't need a molar mass. If you're looking for one, you're completely on the wrong track. $\endgroup$ – tschoppi Dec 9 '14 at 0:29

The equation of the reaction provides you with the composition of the gas liberated.

You assume ideal gas, which means that the gas particles are indistinguishable. This means the reaction equation could look like $$ \ce{2Pb(NO3)2 -> 5X + 4PbO} $$ where 5 moles of gas are liberated for every 2 moles of lead nitrate.

Reforming the ideal gas equation to $n=pV/(RT)$ will lead you to the numerical value. The amount of lead nitrate will then be $$n_{\ce{Pb(NO3)2}} = \frac{2n}{5}$$

From that calculating the mass is a trivial task.

  • $\begingroup$ Thank you for the answer. So you can add the gasses together? That's a new concept, thanks for that. $\endgroup$ – Asker123 Dec 9 '14 at 0:29

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