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Topic: pH of polyprotic acid with other substances  (Read 395 times)

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Offline ubcdumbass

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pH of polyprotic acid with other substances
« on: February 14, 2019, 01:56:56 AM »
Question 6 

Calculate the pH of a solution prepared by mixing equal volumes of 0.050 M sodium oxalate, Na2C2O4 (aq), and 0.010 M ammonium chloride, NH4Cl (aq).

For ammonium, Ka = 5.70 × 10–10; for oxalic acid, Ka,1 = 5.60 × 10–2 and Ka,2 = 5.42 × 10–5.

Hint #1: Use the charge balance equation as your main equation. Use the mass balance and equilibrium equations to find an expression for each species in terms of [H3O+] or [OH–], then substitute into the charge balance equation.

Hint #2: If no simplifying assumptions are obvious, you can always solve the exact final equation numerically using algebra and Excel. See the pre-exercise for a pair of methods.

I got 7.65 and I'm unsure as to why I'm wrong. I tried to calculate the OH- and H30 concentration for the two salts separately and determine pH from that.

Offline Borek

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Re: pH of polyprotic acid with other substances
« Reply #1 on: February 14, 2019, 03:43:11 AM »
I got 7.65 and I'm unsure as to why I'm wrong. I tried to calculate the OH- and H30 concentration for the two salts separately and determine pH from that.

You were told how to solve the problem, you ignored the hint, why are you surprised you got a wrong result?

I tried to calculate the OH- and H30 concentration for the two salts separately and determine pH from that.

It never works, pH is not additive in any way.

This is an amphiprotic salt, final pH depends on several factors that you can't ignore, hence the need for a rather complete approach.
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Offline ubcdumbass

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Re: pH of polyprotic acid with other substances
« Reply #2 on: February 14, 2019, 09:44:12 AM »
I know how to do charge balance and mass balance but I'm not sure how to calculate the pH when it comes to two salts disassociating together.

Offline Borek

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Re: pH of polyprotic acid with other substances
« Reply #3 on: February 14, 2019, 04:43:55 PM »
Write all necessary equations (balances, equilibria) and try to solve them. That's what you are told to do, I still don't understand why you don't follow hints.
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