[chemfreak007]

I know that buffer solution can contain a weak acid or base with is salt.
but is there something im missing?

a)100mL of 0.1M Na2CO3 and 75 mL of 0.2 M HCl
b) 50mL of 0.2M Na2CO3 and 5 mL of 1.0M HCl
c. 100mL of 0.1M NaHCO3 and 25mL of .2M HCl
d) 100mL of 0.1M Na2CO3 and 50 mL of 0.1 M HCl
e) 100mL of 0.1M NaCO3 and 50mL of 0.1M NaOH

out of those 5 choices, I cant find an answer. I think all of them are reasonable to produce a buffered solution.

[Borek]

Do the calculations for each answer and check if you have both acid and conjugated base in the solution after mixing. If not, there is no buffer. Don't forget that in practice there is no such thing as free H2CO3 acid.

So for example a: after mixing you will have solution containing 0.005 mole HCO3(-) - but no H2CO3 (carbon dioxide bubbled out from the solution), so it is not a buffer solution (you have conjugated base HCO3(-) but no acid H2CO3).

And so on.

[GCT]

I know that buffer solution can contain a weak acid or base with is salt.
but is there something im missing?

a)100mL of 0.1M Na2CO3 and 75 mL of 0.2 M HCl
b) 50mL of 0.2M Na2CO3 and 5 mL of 1.0M HCl
c. 100mL of 0.1M NaHCO3 and 25mL of .2M HCl
d) 100mL of 0.1M Na2CO3 and 50 mL of 0.1 M HCl
e) 100mL of 0.1M NaCO3 and 50mL of 0.1M NaOH

out of those 5 choices, I cant find an answer. I think all of them are reasonable to produce a buffered solution.

Which of the bases is considered a weak base?  I would go with c), right off the top.  Is the conjugate acid of this weak base, a weak acid?  Find the Ka of dihydrogen carbonate

[jdurg]

Also remember that HCO3- can be considered a weak acid since an equillibrium exists between HCO3- and CO3(2-).

[Borek]

Find the Ka of dihydrogen carbonate

Perhaps my English fails me... What is the dihydrogen carbonate?

[AWK]

You should start from reaction of sodium carbonate (or bicarbonate) with HCl (or NaOH). Strong acid (HCl) replace a weak one (carbonic acid). Then you should decide which type of calculation you should do - buffer solution, hydrolysis of salt or excess of strong acid (or base). For these calculations you need (probably) both dissociation constants of carbonic acid H2CO3.
THe last (e) problem shows a printing error. NaCO3 cannot exist.

eg - example c
NaHCO3 + HCl = H2CO3 + NaCl

10 milimoles of NaHCO3 + 5 milimoles of HCL =  5 milimoles of H2CO3 + 5 milimoles of excessing NaHCO3 and 5 milimoles of NaCl
Hence, this is a buffer solutions containing the same amounts (or concentrations) of H2CO3 and NaHCO3 and its pH, in this case, is equal to pKa1 of H2CO3 (without taking into account ionic strength)

[Borek]

5 milimoles of H2CO3

I think it bubbled out before you finished writing the answer

So there are no 5 milimoles of acid in the solution. You are left with a mixture of NaHCO3 and NaCl.

Of course the solution will be in the equilibrium with CO2 from the air so some H2CO3 will be present and there will be some buffering effect, but much weaker then one may expect.

[AWK]

I think it bubbled out before you finished writing the answer

Solubility of CO2 in pure water is about 1.8 g/L at room temperature.
Concentration of H2CO3 in example c is 0.04M, something less than 1.8g/L.
Hence, assuming the summary concentration of NaCl and NaHCO3 of 0.08M has negligible influence on solubility of CO2,  all CO2 will be in solution, and my assumption hold water.

[Borek]

Funny thing - seems you are right by about 3.5%

I doubt these numbers were deliberately put into the question, I think they are rather accidentally right.

[GCT]

dihydrogen carbonate, H2C03, wasn't quite sure if this was the proper name though.

[Borek]

dihydrogen carbonate, H2C03, wasn't quite sure if this was the proper name though.

H2CO3 is carbonic acid as far as I am aware off.