[chay722]

A solution is prepared by mixing 20 mL of 1 M HCl with 80 mL of .5 M HNO2. I am asked to calculate the concentration of the NO2-. I know how to find the concentration but I just need th equation of the reaction to start this problem. I am kind of lost. It wouldn't be HCl+HNO2-->, would it? That doesn't really make sense to me. Thanks for any help.

[Borek]

Do you know how to calculate ratio of HNO₂/NO₂^- concentrations knowing pH and pKa?

Hint: rearrange dissociation constant definition.

[chay722]

um yes I believe I do. so should I set up the reaction such that HNO2+ H2O-->H30+ + NO2- . if I do this though, it would be ignoring the HCl in the problem. I'm not sure how to incorporate the two together.

[Borek]

um yes I believe I do.

What if you assume HCl is solely responsible for pH?

Note, that after you will calculate ratio of HNO₂/NO₂^- you can easily check how much H⁺ was produced due to the HNO₂ dissociation - that'll give you an idea whether the initial assumption (that pH is given by HCl concentration only) was OK or not.

[chay722]

How would checking how much H+ was produced due to the HNO2 dissociation show that the pH is given by HCl concentration only?

[Borek]

If it is - say - 2% of amount from HCl dissociation, does it change much?

[chay722]

I'm sorry I meant, how would I find out that 2% of amount was from the HCl dissociation?

[Borek]

No no no, you misunderstood me. If amount of H+ from HNO2 (calculated from ratio)  is 2% of H+ from HCl (calculated from 100% dissociation), you may safely assume HNO2 dissociation doesn't change pH.

[nj_bartel]

Borek - quick question.  Doesn't LeChatlier's come in to play for dissociation of the HNO₂ since it's in such a high [H⁺] already?

[Borek]

Yes, it can be put that way.