# What is the pH given the pKa1 and pKa2?

What is the pH of 0.00109 mol/L H2SO4(aq) at 25oC? For H2SO4, Ka1 is very large and pKa,2=1.96 at 25oC.

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• Dr.A
Lv 7

H2SO4 => H+ + HSO4-

[H+]= 0.00109 M = [HSO4-]

for the second dissociation :

HSO4- <------> H+ + SO42-

initial concentration

0.00109, , , , ,, 0.00109

change

-x .. . . . . . . . . . . . +x. . . .+x

at equilibrium

0.00109-x. . . . .0.00109+x .. . .x

Ka2 =10^-1.96 =0.0110

0.0110 = (0.00109+x)(x) / 0.00109-x

1.20 x 10^-5 - 0.0110 x = 0.00109x+x^2

x^2 + 0.0121x - 1.20 x 10^-5=0

x = - 0.0121 + sq.rt (0.000146 + 0.000048)/2=0.000914

[H+]= 0.000914 + 0.00109=0.00200 M

pH =2.70

Hi Apayan: Virtually the same question was posted four days ago by 'gina":

It is still waiting for an answer. I have been waiting for someone to put it through the quadratic equation soln for me. Now when I went to school (many years ago I admit) we were told it was acceptable to consider H2SO4 fully dissociated in dil soln. What pH does this give us: [H+] = 2.18x10^-3 mol L-1; pH =2.66 A calculation that takes less than a minute, is not prone to error, and is so close to the "correct" answer that it is to be preferred. As I recall pH meters typically have an error of +/-0.02 units below pH 4.0 and pH readings vary with temp. I think there are better things to do to make chemistry relevant and interesting. cheers, drp

• Anonymous
5 years ago

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Wait!!! Given that carbonic acid as H2CO3 doesn't exist, then perhaps you need to rethink this question. What we call carbonic acid is actually a solution of CO2 in equilibrium with H+ and HCO3-. CO2(aq) + H2O <==> H+ + HCO3- ...... Ka = 4.3x10^-7 A buffer is a mixture of a weak acid and its conjugate base. The weak acid is actually aqueous CO2. A solution of NaHCO3 is not by itself a buffer, there must also be some CO2 in solution. The way to do this is to add some H+ to shift the equilibrium to the left, making CO2(aq). The mixture of CO2 and HCO3- and H+ will constitute a buffer.

• 4 years ago

Pka1 Pka2