H2CO3, write the expression for Ka for the acid. Assume only one hydrogen is ionized.
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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.
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