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K a = [ H + ] [ C 2 H 3 O 2 - ] [ HC 2 H 3 O 2 ]

Buffers Buffer - Substances in a solution that resist changes in pH a weak acid and its conjugate salt a weak base and its conjugate salt.

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K a = [ H + ] [ C 2 H 3 O 2 - ] [ HC 2 H 3 O 2 ]

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  1. Buffers Buffer - Substances in a solution that resist changes in pH a weak acid and its conjugate salt a weak base and its conjugate salt

  2. A buffered solution contains 0.50 M acetic acid and 0.50 M sodium acetate. Determine the pH of the solution if the Ka of acetic acid is 1.9 x 10 -5. HC2H3O2 H+ + C2H3O2 - Initial (M) 0.50 0 0.50Change (M) -x x x Equil (M) 0.50 -x x 0.50 + x  .50 Ka = [ H+] [ C2H3O2-] [ HC2H3O2 ] 1.9 x 10 -5 = [ x ] [ 0.50 ] 0.50 x = 1.9 x 10 –5 = H + pH = 4.74

  3. A buffered solution contains 0.50 M acetic acid and 0.50 M sodium acetate. Determine the pH of the solution if the Ka of acetic acid is 1.9 x 10 -5. Ka = [ H+] [ C2H3O2-] [ HC2H3O2 ] 1.9 x 10 -5 = [ x ] [ 0.50 ] 0.50 x = 1.9 x 10 –5 = H + pH = 4.74 Note that the H + concentration depends on the ratio of acid to conjugate in the solution and the Ka If the ratio of acid to conjugate is 1 (equimolar concentrations), then H + concentration equals the Ka

  4. How Does a Buffer Work A strong acid or strong base added to water results in a substantial change in pH since each H + or OH – results in a 1:1 change in [H +] or [OH -]. A buffer solution for example contains a weak acid and its conjugate base: NaC2H3O2  Na + + C2H3O2 - HC2H3O2  H + + C2H3O2 - The significant acid-base species in the buffer are: HC2H3O2 and C2H3O2 -

  5. If an acid is added to the solution: H + + C2H3O2 -  HC2H3O2 No significant change in H + The added H + is sequestered by forming HC2H3O2 If a base is added to the solution: OH - + HC2H3O2  H2O+ C2H3O2 - No significant change in OH – The added OH – is sequestered by forming HOH

  6. The weak acid functions to buffer any hydroxide added to the solution The conjugate base functions to buffer any hydrogen ion added to the solution An effective buffer should have approximately equal concentrations of the weak acid and its salt so that it buffers equally against decreases and increases in pH

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