# What is the relationship between equilibrium concentrations in two similar experiments?

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?

• @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. – Martin - マーチン Sep 27 '17 at 8:00
• @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 ;) – airhuff Sep 27 '17 at 18:06