A chemist needs to prepare a buffer using the carbonate system (information in the table below) with a final pH of 6. Which components should they mix in water to product this buffer?
H2CO3 HCO3- CO32-
pKa 6.35 10.33 N/A
You write out the ionization expression and see what you have.
H2CO3 ==> H^+ + HCO3^-
k1 = (H^+)(HCO3^-)/(H2CO3) = ?
Since you want pH of 6, use the closest pKa value which is 6.35. That dictates you use HCO3 and H2CO3.
To prepare a buffer with a final pH of 6 using the carbonate system, the chemist needs to mix the appropriate amounts of the two components: H2CO3 (carbonic acid) and HCO3- (bicarbonate ion).
The pH of a buffer system is determined by the ratio of the concentrations of its acid and conjugate base. In this case, carbonic acid (H2CO3) acts as the acid, while bicarbonate ion (HCO3-) acts as the conjugate base.
Since the pKa values are given for both components, we can use the Henderson-Hasselbalch equation to determine the ratio of the acid and conjugate base required to achieve a pH of 6:
pH = pKa + log([base]/[acid])
Substituting the given values:
6 = 6.35 + log([HCO3-]/[H2CO3])
Rearranging the equation:
log([HCO3-]/[H2CO3]) = 6 - 6.35
log([HCO3-]/[H2CO3]) = -0.35
Now, we can convert this logarithmic equation into an exponential equation:
[HCO3-]/[H2CO3] = 10^(-0.35)
[HCO3-]/[H2CO3] = 0.447
To achieve a pH of 6, the concentration of bicarbonate ion ([HCO3-]) should be 0.447 times the concentration of carbonic acid ([H2CO3]). However, since the pKa for CO32- is not given, we can assume it is not relevant in this case (it is not necessary to include it in the buffer preparation).
To summarize, the chemist should mix carbonic acid (H2CO3) and bicarbonate ion (HCO3-) in a ratio of 0.447 (or 0.447:1) to prepare a buffer with a final pH of 6.