Gen Chem Problem Help!

[DerekC415]

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I need help with this problem. It was from my chem lab book:

If you mix 10mL of 0.1M sodium acetate with 25mL of 0.2M acetic acid, what will the pH of the resulting buffer be? The molarity?

For molarity I got 0.171. Correct?😕

Since no pKa was provided, I'm hung up on how to find the pH. Can you please walk me through it?

Thank you!

 

[hausee]

You sure you have all the info?

 

[mx41]

Usually the pKa should be given, or at least the Ka. I'm pretty sure you would be given this on the DAT. In any case, acetic acid's Ka is pretty well known (1.8 x 10^-5), so you can use that Ka into the Henderson-Hasselbach equation to find the answer.

 

[Orgolover]

Agreed. I think it was in your lab book because they assumed you would check up the Ka (or pKa for that matter). You would use standard Hendersen hasselbach equation.

Acetic acid is a carboxylic acid which generally have pKa in range of 4-5. This you should know for organic material anyway. So even though the pKa is not give we should be able to get a good idea of what it will be anyhow.

pH = pKa + log(A- / HA)

pH = 4.5 + log (.1/.2) = 4.5 + log (5 x 10^-1) = 4.5 - .3 = 4.2

Should be good enough of an estimate. . .

 

[DerekC415]

Thank you for the help