Two experiments were performed involving the following equilibrium. The temperature was the same in both experiments. $$\ce{H2 (g) + I2 (g) <--> 2HI (g)}$$

In experiment A, $\pu{1.0 M}~\ce{H2}$ and $\pu{1.0 M}~\ce{I2}$ were initially added to a flask and equilibrium was established. In experiment B, $\pu{2.0 M}~\ce{HI}$ was initially added to a second flask and equilibrium was established. Which of the following statements is always true about the equilibrium concentrations?

  1. $\ce{[H2]}$ equals $\ce{[HI]}$ in experiment A.
  2. $\ce{[HI]}$ equals $\ce{2[H2]}$ in experiment A.
  3. $\ce{[HI]}$ in experiment A equals $\ce{[HI]}$ in experiment B.
  4. $\ce{[HI]}$ in experiment A equals 2 $\ce{[I2]}$ in experiment B.

Can someone explain why the answer is 3. Doesn't that mean 2 and 4 are true as well?

  • $\begingroup$ @airhuff Your edit was not only completely superfluous, but also harmful. Please pay attention to buzzwords, and don't introduce them after they have been already removed. Unfortunately I noticed after I spent too much time to polish that thing, that I had done that already. Also note, that the abbreviation for concentration should be all set in math mode, in some browsers it looks terrible otherwise. $\endgroup$ – Martin - マーチン Sep 27 '17 at 8:00
  • $\begingroup$ @Martin-マーチン Yea, agreed. I'd been a member for about 2 weeks at the time I made the edit and have no idea why I thought that was an improvement. Rookie mistake ;) $\endgroup$ – airhuff Sep 27 '17 at 18:06

Note that you are not given the equilibrium constant so you do not know the absolute equilibrium concentration of any species at equilibrium. This eliminates 2 and 4.

Since all conditions and total concentration of reactants + products are the same between the two experiments, you do know that the concentration of any species in experiment A is equal to that species in experiment B. Again, only at equilibrium, and you don't know to what degree that equilibrium lies in terms of absolute concentrations.

  • $\begingroup$ Thank you! I don't have that deep of an understanding so could you please expand? I don't get if the stoichiometric ratio has anything to do with the calculation of HI or any other species in this problem. Or do you need to use ICE charts? $\endgroup$ – A.AK Jan 23 '17 at 6:28
  • $\begingroup$ I hope I can explain. Note that the reaction is written with a 2-way arrow, meaning the reactants and products are in a state of equilibrium. What is the concentration of any species at equilibrium? You can't tell without more information so that wasn't the point of the problem. The point is that it doesn't matter if you start with only the compounds on the right side of the equation or the left. At equilibrium they will come to the same concentration either way. That's how you know C is right; [HI]=[HI], it doesn't matter how you got there. I hope I understood you and I hope that helped! $\endgroup$ – airhuff Jan 23 '17 at 6:46

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