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Topic: PH  (Read 1702 times)

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Offline orgo814

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PH
« on: October 30, 2013, 11:39:06 AM »
This is from a biochemical calculations book but I find it more general chemistry in topic so I'm posting it here

1) what is the pH of a solution containing 0.2 g/L Na2CO3 and 0.2 g/L NaHCO3? (pKA2= 10.25)

In struggling with this one. Using Henderson hasselbach equation, I figured the concentration ratio would be 1, making the log of it 0 and then pH=pKa= 10.25. According to the book it is 8.28 and I can't figure out why

2) what volume of glacial acetic acid (density=1.06 g/mL) and what weight of solid potassium acetate are required to prepare 5 liters of 0.2 M acetate buffer, pH= 5?

I used Henderson hasselbach to find the ratio of base/acid and then used that to find a mole fraction. I multiplied mole fraction by concentration to get moles of each component but did not get correct answer after going from there. Any help appreciated

Offline magician4

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Re: PH
« Reply #1 on: October 30, 2013, 11:52:06 AM »
ref 1)
use molar ratios, not mass ratios, with HH !

ref 2)
Quote
I used Henderson hasselbach to find the ratio of base/acid and then used that to find a mole fraction. I multiplied mole fraction by concentration to get moles of each component but did not get correct answer after going from there. Any help appreciated
as this sounds like a good approach sofar, pls. expand on what exactly you did, i.e. show your calculation, so we could look for errors


regards

Ingo
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