Final answer:
The fraction of carbon dioxide present as carbonic acid is calculated by dividing the concentration of carbonic acid by the total concentration of carbonic acid and bicarbonate ion, which is 4.8% when rounded to two significant figures.
Step-by-step explanation:
To calculate the fraction of carbon dioxide present as carbonic acid, we can use the known concentrations of carbonic acid (H2CO3) and the bicarbonate ion (HCO3−). The concentration of carbonic acid is 0.0012 M, and the concentration of bicarbonate ion is 0.024 M. According to the given information, we should use the Henderson-Hasselbalch equation:
pH = pKa + log([base]/[acid])
However, since we are not asked to calculate pH but the fraction of H2CO3, we can simply use the given concentrations to find the ratio directly:
Fraction of H2CO3 = [H2CO3] / ([H2CO3] + [HCO3−])
Substituting in the given values:
Fraction of H2CO3 = 0.0012 M / (0.0012 M + 0.024 M)
Fraction of H2CO3 = 0.0012 M / 0.0252 M = 0.0476, or 4.76% when expressed as a percentage.
To two significant figures, the fraction of CO2 present as carbonic acid is 4.8%.