Calculate the H30+ of the following polyprotic acid solution: 0.130 M H3C6H5O7 Express your answer using two significant figures. Calculate the pH of this solution. Express your answer to two decimal places.
The Ka values for the citric acid are:
For brief notation I'll write the acid as
The solution is made with the fully protonated acid H3Cit, this means the first dissociation is going to happen,
And since the concentration of the acid is high, the main contriubution to the pH is given by the first dissociation:
You need to analize the reaction with an ICE table
H3Cit | ----> | H+ | H2Cit | |
Initial | 0.130M | |||
Change | -X | +X | +X | |
Equilibrium | 0.130-X | X | X |
Using the definition of the Ka for the first dissociation you can solve this:
and plugging the concentrions in the equilibrium in this
equation:
This yields a second degree equation,
X2+7.1x10-4X-(0.13)(7.1x10-4)=0, can be solved with the quadratic formula, and yields
X=9.26x10-3M =[H+] H+ is the same as H3O+
pH=-log[H+]=-log(9.26x10-3)=2.03
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