0070 mol 10 l 0070 m 00330 mol 10 l 0330 m x x 00070

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Unformatted text preview: HClO(aq) + OH–(aq) → When we add NaOH (strong base), it fully dissolves and dissociates. The OH– will react with HClO, reducing [HClO] and increasing [ClO–]. HClO(aq) + NaOH(aq) ← Na+(aq) + ClO–(aq) + H2O → HClO Initial 0.0100 Mx1.0 L = 0.0100 mol Change – 0.0030 mol Final 0.0100 – 0.0030 = 0.0070 mol OH– 0.0030 mol – 0.0030 mol 0 ClO– 0.0300 Mx1.0 L = 0.0300 mol + 0.0030 mol 0.0300 +0.0030 = 0.0330 mol [0.0330] pH = pKa + log [Base] = –log(3.5 × 10–8) + log = 8.129 [0.0070] [Acid] 18 Friday, March 1, 13 Calculation Time Initial composition Change in composition Equilibrium composition HClO ClO– 0.0070 mol / 1.0 L = .0070 M 0.0330 mol / 1.0 L = .0330 M –x +x 0.0070 – x ≈ 0.0070 [H O+][ClO–] 0.0330x Ka = 3 = 0.0070 [HClO] = 3.5 × 10–8 0.0330 + x ≈ 0.0330 x= H3O+ 0 +x x (3.5 × 10–8)(0.0070) = 7.4 × 10–9 0.0330 x << 0.0070 and 0.0330. HH is ok [H3O+] = x = 7.4.. × 10–9 pH = –log [H3O+] = –log(7.4.. × 10–9) = 8.129 As long as x is small enough, then the HH equation gives the same answer as the full ICE table. 19 Making Buffers (theoretically) pH = pKa + log [base]in [acid]in The simplest way to make a buffer for a specific pH, is to find an acid with a pKa equal to the pH you need, and then mix acid and conjugate base in a ratio of 1:1; i.e., equimolar amounts of acid and conjugate base. Because log 1 = 0, so pH = pKa when [base]in = [acid]in If you can’t find an acid with a pKa equal to the pH you need, then you use one that is close to your desired pH and then add a little extra acid or conjugate base (tuning the 1:1 ratio) to make the final pH where you wanted it to be. 20 Making Buffers (practically) The HH equation is based on approximations, and also describes an “ideal” situation (meaning there are more physical effects we are not taking into account, that you’ll learn in more advanced classes). In practice in the lab, we use HH to figure out how to make a buffer with a pH close to the desired value. If you make that buffer exactly as you calculate it, the experi...
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This note was uploaded on 04/09/2013 for the course CHEM 302 taught by Professor Mccord during the Spring '10 term at University of Texas.

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