[PharmD]

In a situation of a mixture between a strong acid (hcl) and a weak acid (h2co3), how large does the strong acid concentration needs to be, for us to dis-consider the weak acid dissociation? And what about the weak acid concentration, isnt it also important?

Thanks.

[Borek]

I don't think there is a simple answer to that question. First of all - it is way too general and ambiguous.

[PharmD]

Well, if I was expecting a simple answer, i wouldn't come to you guys 

And what about considering a mixture of 25 ml hcl 0,05M plus 25 ml of carbonic acid 0,1M. How would you calculate the ph?
I know that this concentration of hcl limits almost completely the h2co3 dissociation, but i was looking for a threshold concentration value of hcl that might be commonly used in undergraduate teaching for simplification.

PS: this is more of an analytic chemistry question  .

[Borek]

I doubt you will find a single number, at best it will be a function of weak acid concentration and first Ka value.

And when you mix carbonic acid with strong acid you need to account for decomposition and carbon dioxide evolution, and the final answer depends on the partial pressure of CO₂ (although it is very likely that in 0.05M HCl it can be safely ignored).

I guess something like calculating H⁺ concentration in a solution of a weak acid, then assuming twentyfold concentration of a strong acid should work. Why 20? To use 5% rule. But that's where I would look for a solution, doesn't mean exact analysis would not yield better rule.

[PharmD]

Useful reply. Thanks for your help borek. 

[AWK]

And what about considering a mixture of 25 ml hcl 0,05M plus 25 ml of carbonic acid 0,1M

Concentration of 0.1 M CO₂ (not carbonic acid) in water is possible for p_CO2 over 3 atm.