H2CO3, write the expression for Ka for the acid. Assume only one hydrogen is ionized.
The expression Ka is defined as the acid ionization constant. It is the equilibrium constant for the dissociation of an acid into its resulting ions. As a result, it is basically a numerical measure of the strength of an acid. In general, an acid HA will dissociate into its resulting ions:
HA <--> H+ + A-
The Ka expression for this general expression is:
Ka = ([H+][A-])/[HA]
where the terms in the brackets  are the concentration values for the different species. H2CO3 is called carbonic acid and its first acid dissociation is written below:
H2CO3 <--> H+ + HCO3-
As a result, the Ka expression is:
Ka = ([H+][HCO3-])/[H2CO3]
It should be noted that carbonic acid is a weak acid, so its Ka value in water is only 4.3 x 10^-7. This small a value means that the denominator is much larger than the numerator in the Ka expression which makes sense for a weakly ionizing acid.