Using the Equilibrium Constant to Determine Quantities

There are two common variations on the type of equilibrium problem in which we solve for K one involves a straightforward substitution of quantities, and the other requires first finding some of the quantities. 

Substituting Given Equilibrium Quantities into Q to Find K In the straightforward case, we are given the equilibrium quantities and we must calculate K. 

Suppose, for example, that equal amounts of gaseous hydrogen and iodine are injected into a 1,50-L reaction flask at a fixed temperature. In time, the following equilibrium is attained  

At equilibrium, analysis shows that the flask contains 1.80 mol of H2, 1.80 mol of I2, and 0.520 mol of HI. We calculate Kc by finding the concentrations and substituting them into the reaction quotient. Given the balanced equation, we then write the reaction quotient  

We first have to convert the amounts (mol) to concentrations (mol/L), using the flask volume of 1.50 L  

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