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Help on Biochemistry hw

MyPinkBunny
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Help on Biochemistry (Buffers and pH)

 

The buffer is 4 mL of 0.1M Tris, pH 9 undiluted. We will add 1 mL of 0.05 N HCL. What is the expected pH.

 

I took the henderson-hasselbach equation pH = pka + log [base]/[acid] and plugged in pka = 8.21 and [base]=0.1 M and [acid]=.05 M and got a pH of 8.51 but apparently that was wrong. The answer is 8.65. I have no idea how to arrive at 8.65. I'm guessing you have to take into account the 4 mL and the 0.05 N HCL, but I don't know how to do that. Please can anyone help guide me in the right direction? I've been working on this problem for hours and our class doesn't have a textbook with examples. Thank you!

What you have to take into account is the final concentration in your solution. So you have to take the dilution into account as a result of mixing. Hint1: the final volume is 5 ml.

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