[tarquin]

Hello,
I am trying to find the concentrations of two solutions based on their pH for a titration. One is HCl with a pH 0.7and the other is Na₂CO₃ with a ph of 10. Here is my attempt:

HCl:

It is a strong acid so it will dissociate 100% so the concentration can be found using this equation:
HCl + H₂O -> Cl^- + H₃O⁺
[H3O] = 10^-0.7M = [HCl]

NaCO₃:
CO₃^2- + H₂O -> HCO₃⁺ + OH^- *note: I took out the Na, because i think it was a spectator ion, could this be where I went wrong*

ICE table

  CO₃^2-  HCO₃⁺    OH^-
I    C_i       0       0
C   (-x)   (+x)  (+x)
E  (C_i-x)   (x)   (x)

x = 10^-4 (This came from it having a pH of 10 so 14-10 = pOH of 4)
(10^-4)²/(Ci-10^-4) = kb = 2.13*10^-4
Ci of CO₃= 1.43 *10 ^-4 M

This number seems to be very low, but I am not quite sure where I went wrong...
Thanks!

[Borek]

This is correct value, but using pH to find concentration is very unreliable.

[tarquin]

I didn't factor this in, but CO₃ is polyprotic, does that make a difference?

[tarquin]

What I mean is that it reacts with water to form HCO₃^- and then that reacts to form H₂CO₃. Does this affect my calculations? HCO₃^- > H₂CO₃ has a Ka value of 4.5x10^-7, so a Kb of 2.23 *10^-7

[Borek]

You an safely ignore H₂CO₃, HCO₃^- is a much weaker base than CO₃^2-.