patino_chm2046_chapter16

30 or h3o 50 10 5 m h 50 possible acidsbases hso4

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Unformatted text preview: sic pH (e.g. 10.13) we need to use a For base B with pKb about equal to 3.87 (because pOH =3.87) and a salt containing its conjugate acid BH+ =3.87) Preparing a Buffer We want to buffer a solution at pH = 4.30 or [H3O+] = 5.0 × 10-5 M [H 5.0 POSSIBLE ACIDS/BASES HSO4− / SO42HSO SO HOAc / OAcHOAc HCN / CNHCN -Choose Ka 1.2 x 10-2 -5 √ 1.8 x 10 4.0 x 10-10 an acid such that Ka ≈ [H3O+] (or pKa ≈ pH). (or Best choice is acetic acid / acetate ion. Preparing a Buffer You want to buffer a solution at pH = 4.30 or [H3O+] = 5.0 × 10-5 M. With acetic acid, Ka=1.8x10−5 [H 5.0 With + ] = 5.0 x 10-5 = [HOAc] (1.8 x 10 ) -5 [H3O - [OAc ] Solve for Solve [HOAc] - ] ratio = 2.78 [OAc 2.78 = 1 Therefore, if you use 0.100 mol of NaOAc and 0.278 Therefore, mol of HOAc in 1.00 L, the pH = 4.30. mol [HOAc] = 2.78 x [OAc−] = 2.78 x 0.100 = 0.278 M Effectiveness of Buffers The relative concentrations of acid & base should not differ by a more than a factor of 10 to be reasonably effective; [base]/[acid] = 10 or [base]/[acid] = 0.1 • In fact, a buffer will be most effective when the [base]:[acid] = 1 that is, equal concentrations of acid and base a) [acid]=[base] = 0.5 M b) [acid]=[base] = 0.1 M • Also, a buffer will be most effective when the [acid] and the [base] are large 25 Buffering Range for an effective buffer, 0.1 < [base]:[acid] < 10 for • we can use Henderson-Hasselbalch eqn to calculate the maximum and minimum pH over which the buffer will be effective [A - ] pH = pK a + log [HA] Lowest pH pH = pK a + log( 0.10 ) pH = pK a − 1 Highest pH pH = pK a + log(10 ) pH = pK a + 1 therefore, the effective pH range of a buffer is pKa ± 1 when choosing an acid to make a buffer, choose one whose pKa is the closest to the pH of the buffer 26 Which of the following acids would be the best choice to combine with its sodium salt to make a buffer with pH 4.25? Chlorous Acid, HClO2 Nitrous Acid, HNO2 Formic Acid, HCHO2 Hypochlorous Acid, HClO pKa = 1.95 pKa = 3.34 pKa = 3.74 pKa = 7.54 The pKa of HCHO2 is closest to the desired pH of the buffer, so it would give the most effective bu...
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