pH question

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Marina Gollas 1A
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Joined: Fri Sep 28, 2018 12:20 am

pH question

Postby Marina Gollas 1A » Thu Mar 14, 2019 4:25 am

Sodium Benzoate (NaC7H5O2) is used as a food preservative. What is the pH of a 0.10 M solution of sodium benzoate? The pka of benzoic acid (C2H6O2) is 4.404. Can someone help break this up for me? :) Thank you.

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Joined: Fri Sep 28, 2018 12:18 am

Re: pH question

Postby 705192887 » Thu Mar 14, 2019 12:43 pm

First, write out the balanced reaction: C7H5O2 + water --> C7H6O2 + OH-. This reaction is basic, so you need to convert pKa into pKb to use for this reaction. 14-pKa=pKb, so pKb = 9.596. Next, convert pKb into Kb in order to use it in an ice table. 10^-pKb = Kb = 2.535x10^10. Now, from an ice table, Kb=[x]^2/(.1-x). Using the newfound Kb value and the quadratic formula, x is solved to be 5.0347x10^-6. This value is [OH] concentration. 14-pOH equals pH, so 14-(-log[OH]) = 14-(-log 5.0347x10^-6) leaves you with a pH value of 8.7. This value makes sense because the reaction is basic and a pH of 8.7 is greater than 7 and therefore basic. hope this makes sense and helps!!

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