Question

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.

Answer 1

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