Buffer Solutions?

Question

Can someone please explain to me how to find the pH of a buffer solution? I don't understand at all.

If you can walk me through this example, that'd help a lot.

Calculate the pH of a buffer that is 0.0858 M in NaHCO3 and 0.120 M in Na2CO3.

I think this is the right Ka value:
Ka for H2CO3
Ka1 =4.3E-7 Ka2 =5.6E-11


Thank you so much in advance...I really need to understand this..

Also:

Calculate the pH of a solution formed by mixing 78.0 mL of 0.0858 M NaHCO3 with 28.0 mL of 0.120 M Na2CO3.


HELP!!!!

Answer 1

Sorry Phil, but you used the wrong Ka value. Since the buffer is between HCO3- and CO3(2-), Ka2 is used, not Ka1.

pH = pKa + log [base]/[acid]
pH = 10.25 + log (0.120/0.0858) = 10.25 + 0.15 = 10.40

For your second part, which was "Calculate the pH of a solution formed by mixing 78.0 mL of 0.0858 M NaHCO3 with 28.0 mL of 0.120 M Na2CO3," you need to figure out the number of moles of each and divide by the total volume to get the new concentrations, then plug that back into Henderson-Hasselbach to figure out the pH.

[HCO3-] = 0.0858 M * 78 ml/106 ml = 0.0631 M
[CO32-] = 0.120 M * 28 ml/106 ml = 0.0317 M

pH = 10.25 + log (0.0317/0.0631) = 10.25 - 0.30 = 9.95

Answer 2

for bufffers use this formula:

pH=pKa+log([base]/[acid])

so in this example

pKa= -log(4.3E-7)
pKa=6.37

pH=6.37+log(.0858/.120)
pH=6.22