Henderson Hasselbach question

[tcon91]

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Hey everyone!

I'm trying to practice manipulating and using the Henderson-Hasselbach equation so I've been making up my own problems and trying to solve them. Can someone solve this problem below?

Find the pH of a solution that has a pka of 6.4 and Ka of 4 x 10^-7

pka + log ( [A-]/[HA])= pH

I got an answer of -1.1 but I don't think this is correct. Can someone help?

 

[Rhino1000]

Ka=[H+][A-]/[HA]=X^2/1-x=X^2/1=X^2, therefore 4*10^-7=X^2, and X^2 = 40*10^-8, so X = 2sqrt(10)*10^-4, and -log of X equals -log (about 6.2*10^-4) = -log3-log2+4=-.47-.3+4=3.2
Plus or minus .2?

-log X is the same thing as -log[H+] = pH.

 

[tcon91]

yup I believe you are right. Thank you!