Barrons SAT Subject Test Chemistry, 13th Edition

[I 2 ] = (3.00 – x)

mol/L

(the same x is used since 1 mol of H 2 requires 1 mol of I 2 to

react to form 2 mol of HI)

[HI] = 2x mol/L

Then

If

then taking the square root of each side gives

Solving for x:

x = 2.32

Note: On the SAT Subject Test in Chemistry, calculators are not permitted.

So answers to questions like this would be easily calculated “perfect

squares.”

Substituting this x value into the concentration expressions at equilibrium we have

The crucial step in this type of problem is setting up your concentration
expressions from your knowledge of the equation. Suppose that this problem had
been:

Second Sample Problem

Find the concentrations at equilibrium for the same conditions as in the preceding
example except that only 2.00 moles of HI are injected into the box.