Calculating ratio of bicarbonate ion to carbonic acid in blood For the following reaction, $\ce{K_a} = \pu{4.2e-7}$ :

$\ce{H2CO3(aq) + H2O(l) &lt;=&gt; H3O+(aq) + HCO3-(aq)}$

We're given the pH of blood, which is $7.4$. We need to calculate the ratio ("$X$") of bicarbonate ion to carbonic acid in blood.

I know I need to convert the $\ce{K_a}$ to $\ce{pK_a}$, and did so as follows:

$\ce{pK_a} = -\log(\ce{K_a}) = -\log(\pu{4.2e-7}) = 6.4$

But I'm not sure how to proceed from here. Any help would be greatly appreciated.
For these types of problems, the general approach would be to use the Henderson-Hasselbalch equation. For a general weak acid/conjugate base system, this has the form: $$\mathrm{pH}=\mathrm pK_\mathrm a+\lg\left(\frac{\ce{A-}}{\ce{HA}}\right)$$ Since you are given $\mathrm{pH}$ and have found $\mathrm pK_\mathrm a$, it won't require much more work to obtain the ratio.