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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