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Fish200398

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CH3COOH + KOH whats pH?

« on: November 28, 2019, 07:31:59 am » 0,5 M KOH + 4,5 CH3COOHAt equlibrium = CH3COOH left 4MProduce CH3COOK 0,5 MH2O 0,5MKa = 2x10-51. [H] = Ka [CH3COOH] : [CH3COOK]2. The left CH3COOH 4M react with waterCH3COOH + H2O = CH3COO- + H3O+In the end we get, 4M CH3COOH and x mol CH3COO- and H3O+x = [H]

• ^2 = Ka . [4M]

3. salt CH3COOK react with waterCH3COOK + H2O = CH3COO- + K+CH3COO- + H2O = CH3COOH + OH-We left with 0,5 M CH3COO- and y mol of CH3COOH and OHy = [OH]Kb = [CH3COOH][OH] / [CH3COO-][y]^2 = Kb.[CH3COO-] = Kw/Ka . 0,5 MpH = 14-pOHFrom 3 methods, i get 3 different pH.. what is wrong with my calculations? Logged

AWK

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Re: CH3COOH + KOH whats pH?

« Reply #1 on: November 28, 2019, 07:46:31 am » Forget equations 2 and 3. Just insert your concentrations and Ka into equation 1 (Henderson-Hasselbach equation). You will get pH below 4. Logged AWK

Fish200398

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Re: CH3COOH + KOH whats pH?

« Reply #2 on: November 28, 2019, 08:29:22 am » But why 2 and 3 is wrong? Logged

mjc123

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Re: CH3COOH + KOH whats pH?

« Reply #3 on: November 28, 2019, 10:32:05 am » 2. You don't have x M acetate. You have (4-x) M AcOH and (0.5+x) M AcO-. Since x is small, this is equivalent to method 1.You can't ignore the presence of the KOAc and try and work it out for the AcOH alone.3 is wrong for the same reason. Logged

AWK

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Re: CH3COOH + KOH whats pH?

« Reply #4 on: November 28, 2019, 01:21:45 pm » Quote from: Helly on November 28, 2019, 08:29:22 am

But why 2 and 3 is wrong?

There are three basic types for calculating acid-base equilibria: 1) acid or base, 2) buffer solution and 3) salt hydrolysis. Before you start calculating you must decide (correctly) which system you are dealing with. Logged AWK

Borek

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Re: CH3COOH + KOH whats pH?

« Reply #5 on: November 28, 2019, 02:53:00 pm » To add to what mjc123 said: each of these methods is based on some simplifying assumptions. You need to understand where these assumptions come from and when they hold. If they don't hold, method produces wrong results.Compare http://www.chembuddy.com/?left=pH-calculation&right=general-pH-calculation (and derivations of specific cases on other site pages). Logged Chembuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

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