pH calculation (1 Viewer)

[skillstriker]

Calculate the pH of a 0.1 M soln of carbonic acid if only 8% ionises. I keep getting pH = 1.8 but the answer is pH = 2.1 (they didn't double the hydrogen ion concentration). Is the answer wrong or are you not meant to double the hydrogen ion concentration?

 

[Parvee]

Degree of Ionisation = [H+]/concentration
Therefore [H+]= Degree of Ionisation x Concentration
= 0.08 x 0.1
=0.008

pH=-log([H+])
=-log(0.008)
=2.1

 

[skillstriker]

Carbonic acid (H2CO3) is diprotic though. H2CO3 <--> 2H(+) + CO3(2-). So shouldn't you double the [H+]? i.e. [H+] = 0.08 x 0.1 x 2

 

[madharris]

carbonic acid (h2co3) is diprotic though. h2co3 <--> 2h(+) + co3(2-). So shouldn't you double the [h+]? I.e. [h+] = 0.08 x 0.1 x 2

I always thought it was

H2CO3 <--> H+ + HCO3-


Howcome my capital letters aren't working
EDIT: nevermind! it worked

 

[RealiseNothing]

I always thought it was

H2CO3 <--> H+ + HCO3-

^^^

carbonic acid is a weak acid and won't fully dissociate into what you had.

 

[bleakarcher]

^^^

carbonic acid is a weak acid and won't fully dissociate into what you had.

Does that mean that no HCO3- ionisation occurs?

 

[HeroicPandas]

Does that mean that no HCO3- ionisation occurs?

I think that the degree of ionisation (%) for HCO3- will be uber low and would occur

 

[RealiseNothing]

Does that mean that no HCO3- ionisation occurs?

I should have been more clear, my post was at this post:

Carbonic acid (H2CO3) is diprotic though. H2CO3 <--> 2H(+) + CO3(2-). So shouldn't you double the [H+]? i.e. [H+] = 0.08 x 0.1 x 2

I was actually just agreeing with madharris with the "^^^" then went on to say why the OP's equation is wrong lol.

 

[RealiseNothing]

Does that mean that no HCO3- ionisation occurs?

I think that the degree of ionisation (%) for HCO3- will be uber low and would occur

To clarify this anyway, the amount of HCO3- in the solution will depend on which reaction the equlibrium favours. If heaps of H2CO3 was suddenly added, then the equilibrium would shift right and the concentration of the HCO3- would be high.

In terms of the question, only 8% ionises, so it's a very low amount of HCO3- concentration in the solution.