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.
  $$\ce{H2 (g) + I2 (g) &lt;--&gt; 2HI (g)}$$
  
  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?
  
  
  $\ce{[H2]}$ equals $\ce{[HI]}$ in experiment A.  
  $\ce{[HI]}$ equals $\ce{2[H2]}$ in experiment A.  
  $\ce{[HI]}$ in experiment A equals $\ce{[HI]}$ in experiment B.  
  $\ce{[HI]}$ 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?
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.