**Practice Problem 4**

Calculate the rate at which HI disappears in the following reaction at the moment when
I_{2} is being formed at a rate of 1.8 x 10^{-6} moles per liter per
second:

2 HI(*g*) H_{2}(*g*) + I_{2}(*g*)

**Solution**

The balanced equation for the reaction shows that 2 moles of HI disappear for every
mole of I_{2} formed. Thus, HI is consumed in this reaction twice as fast as
I_{2} is formed:

Students sometimes get the wrong answer to this exercise because they become confused about whether the equation for the calculation should be written as:

or as:

You can avoid mistakes by checking to see whether your answer makes sense. The
balanced equation states that 2 moles of HI are consumed for every mole of I_{2}
produced. HI should therefore disappear (3.6 x 10^{-6} *M*/s) twice
as fast as I_{2} is formed (1.8 x 10^{-6} *M*/s).