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Pharmaceutical Analytical Chemistry 213 PHC

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Page 1: Calculate the pH of salt of weak acids. Calculate the pH of salt of weak bases. Define buffer, buffer ratio, and buffer capacity. Calculate

Pharmaceutical Analytical Chemistry

213 PHC

Page 2: Calculate the pH of salt of weak acids. Calculate the pH of salt of weak bases. Define buffer, buffer ratio, and buffer capacity. Calculate

Salt of Weak Acids and Bases

&Buffers

By the end of this lecture you will be able to:

Calculate the pH of salt of weak acids. Calculate the pH of salt of weak bases. Define buffer, buffer ratio, and buffer

capacity. Calculate the pH of weak acid and

weak base buffers.

Page 4: Calculate the pH of salt of weak acids. Calculate the pH of salt of weak bases. Define buffer, buffer ratio, and buffer capacity. Calculate

Salts of WeakAcids and Bases

Page 5: Calculate the pH of salt of weak acids. Calculate the pH of salt of weak bases. Define buffer, buffer ratio, and buffer capacity. Calculate

Salts of weak acids

e.g. NaOAc (sodium acetate) It is completely ionizes It is a Bronsted base It is hydrolyzes in water

OAc- + H2O HOAc + OH-

Kb = [HOAc] [OH-]\ [OAc-]

Page 6: Calculate the pH of salt of weak acids. Calculate the pH of salt of weak bases. Define buffer, buffer ratio, and buffer capacity. Calculate

Cont.

A- + H2O HA + OH-

salt weak acid

= Kw

\ Ka = [HA] [OH-] \ [A-]

Page 7: Calculate the pH of salt of weak acids. Calculate the pH of salt of weak bases. Define buffer, buffer ratio, and buffer capacity. Calculate

pH Calculation

Calculated as weak bases

Kb = Kw \ Ka = [x] [x] \ CA-

Page 8: Calculate the pH of salt of weak acids. Calculate the pH of salt of weak bases. Define buffer, buffer ratio, and buffer capacity. Calculate

Salts of weak bases

e.g. NH4Cl (ammonium chloride) It is completely ionizes It is a Bronsted acid It is hydrolyzes in water

NH4+ + H2O NH4OH + H+

Ka = [NH4OH] [H+] \ [NH4+]

Page 9: Calculate the pH of salt of weak acids. Calculate the pH of salt of weak bases. Define buffer, buffer ratio, and buffer capacity. Calculate

Cont.

BH+ + H2O B + H3O+

salt weak base

Ka = Kw \ Kb = [B] [H3O+] \ [BH+]

Page 10: Calculate the pH of salt of weak acids. Calculate the pH of salt of weak bases. Define buffer, buffer ratio, and buffer capacity. Calculate

pH Calculation

Calculated as weak acids

Ka = Kw \ Kb = [x] [x] \ CBH+

Page 11: Calculate the pH of salt of weak acids. Calculate the pH of salt of weak bases. Define buffer, buffer ratio, and buffer capacity. Calculate

Questions

Page 12: Calculate the pH of salt of weak acids. Calculate the pH of salt of weak bases. Define buffer, buffer ratio, and buffer capacity. Calculate

Buffers

Page 13: Calculate the pH of salt of weak acids. Calculate the pH of salt of weak bases. Define buffer, buffer ratio, and buffer capacity. Calculate

Definition A solution that resists change in pH

when a small amount of an acid or base is added or when the solution is diluted

It consists of a weak acid and its conjugate base or a weak base and its conjugate acid

A mixture of a weak acid and its salt or a weak base and its salt

Page 14: Calculate the pH of salt of weak acids. Calculate the pH of salt of weak bases. Define buffer, buffer ratio, and buffer capacity. Calculate

Buffer of weak acids

HA H+ + A-

pH = pKa + log [A-] / [HA](Henderson-Hasselbalch equation)

Page 15: Calculate the pH of salt of weak acids. Calculate the pH of salt of weak bases. Define buffer, buffer ratio, and buffer capacity. Calculate

Problem 1:

Calculate the pH of a buffer solution containing 0.1M acetic acid and 0.1M sodium acetate (Ka = 1.75 x 10-5).

Page 16: Calculate the pH of salt of weak acids. Calculate the pH of salt of weak bases. Define buffer, buffer ratio, and buffer capacity. Calculate

Buffer of weak bases

BH+ B + H+

pH = pKa + log [B] / [BH+]

pH = (pKw – pKb) + log [B] / [BH+]

pOH = pKb + log [BH+]/ [B]

Page 17: Calculate the pH of salt of weak acids. Calculate the pH of salt of weak bases. Define buffer, buffer ratio, and buffer capacity. Calculate

Problem 2:

Calculate the pH of a solution containing 0.07M NH3 and 0.08M NH4Cl (pKb = 4.76)

Buffer ratio

The ratio of salt / acid

[salt] / [acid] = 1pH = pKa

By changing the buffer ratio, buffers

of different pH values can be prepared

Page 19: Calculate the pH of salt of weak acids. Calculate the pH of salt of weak bases. Define buffer, buffer ratio, and buffer capacity. Calculate

Buffer capacity It is the number of moles /liter of strong

acid or base needed to change the pH of buffer solution

ß = dCacid or base/dpH The buffering capacity is maximum at:

pH = pKa pOH = pKb

It is satisfactory over a pH range of pKa ± 1

The larger it is, the more resistant the solution to pH change

Page 20: Calculate the pH of salt of weak acids. Calculate the pH of salt of weak bases. Define buffer, buffer ratio, and buffer capacity. Calculate

Questions

Page 21: Calculate the pH of salt of weak acids. Calculate the pH of salt of weak bases. Define buffer, buffer ratio, and buffer capacity. Calculate

Summary

pH Calculation of salts of weak acids and bases.

Buffer Definition, buffer ratio, and buffer capacity.

pH Calculation of weak acid and weak base buffers.

Page 22: Calculate the pH of salt of weak acids. Calculate the pH of salt of weak bases. Define buffer, buffer ratio, and buffer capacity. Calculate

Thank you

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