[spunkylulu]

Can someone explain to me how I should go about solving this problem?

What is the pH of a solution of 100 mL of .1M Na₂CO₃ and 50 mL of .1M HCl?
When Ka1= 4.2 X 10^-7 and Ka2= 4.8X10^-11

If someone could explain the reactions to me, that would be most helpful.

[Borek]

What happens to the solution of carbonate, when you add strong acid (hint: take a look at stoichiometry).

[spunkylulu]

Na₂CO₃ +HCl --> 2NaCl +HCO₃?

[Borek]

Check the stoichiometry - do you have enough HCl for the reaction to proceed as written?

I assume HCO₃ to be just a typo?

[spunkylulu]

I don't know.  I have trouble with reactions involving polyprotic acids and bases.  Can you point me in the right direction?

[Borek]

What is bicarbonate?

[spunkylulu]

HCO₃

[Borek]

No, there is no such thing as HCO₃. There is a HCO₃^- ion, which is a first product of CO₃^2- protonation.

Now think about what is happening when you add HCl to teh solution of carbonate. As I said earlier, think about stoichiometry - compare amount of carbonate and amount of HCl.

[spunkylulu]

Okay wait. 

2HCl + CO₃ --> CO₂ + H₂O + 2Cl? 

I feel like I'm not getting anywhere!

[Borek]

2HCl + CO₃ --> CO₂ + H₂O + 2Cl?

Don't ignore charges in your posts.

I told you to watch stoichiometry. Do you have enough HCl for the reaction you wrote?

[spunkylulu]

OH.

No, because the ratio of HCl to Na₂CO₃ is 2:1 and we only have 50 ml of HCl which is only half the amount of Na₂CO₃

[Borek]

So is it possible for the reaction to proceed as you wrote it, or does it have to stop earlier?

[spunkylulu]

No.  So is that why there are two dissociation constants?  But I'd only use the first one? 

[Borek]

We will get to dissociation constants later, for now we need to establish what happened in the solution. And technically solution you got is not different from the one prepared by mixing sodium bicarbonate with equimolar amount of NaCl. Do you see it?

[spunkylulu]

Wait, will this reaction be:

Na₂CO₃ + 2HCl --> 2NaCl + H₂CO₃?