Weak acids and bases Salt of weak acid and bases buffer Lecture 9 9 Feb 2011. Noorulnajwa Diyana Yaacob [email protected] Weak Acid. A weak acid is an acid that does not completely donate all of its hydrogens when dissolved in water.
Weak acids and basesSalt of weak acid and basesbufferLecture 99 Feb 2011
Calculate the pH and pOH of a
1.0 x 10-3 solution of acetic acid
HOAc H+ + OAc-
Ka= [H+][OAc-] = 1.75 x 10-5
x2 = 1.75 x 10-5
1.0 x 10-3 - x
x is smaller than C, neglect it, therefore,
x2 = 1.75 x 10-5 = 1.32 x 10-4M = [H+ ]
1.0 x 10-3
pH = -log 1.32 x 10-4 = 3.88
pOH = 14.00 – 3.88 = 10.12
Ka =[H+][OAc-] = 1.75 x 10-5
Calculate the pH of a 0.10 M solution of sodium acetate
Since COAc > Kb , neglect x compared to COAc. Then,
CH3COOH (aq)CH3COO‒ (aq) + H+ (aq)
It is known as an acidic buffer solution and it maintains a pH value that is less than 7.
NH3 (aq) + H2O(l) NH4+ (aq) + OH‒ (aq)
It is known as a basic buffer solution and it maintains a pH value that is greater than 7.
Calculate the pH of a buffer prepared by adding
10 mL of 0.10 M acetic acid to 20 mL of 0.10 M
0.10 mmol/mL X 10 mL = MHOAc X 30 mL
MHOAc = 0.033 mmol/mL
For OAcˉ ,
0.10 mmol/mL X 20 mL = MOAcˉ x 30 mL
MOAcˉ = 0.067 mmol/mL
= 4.76 + log 2.0
pH = constant + log [A⁻]
*if solution is diluted the ratio remains constant
So, the pH of the solution does not change.
HA H⁺ + A⁻
β = dCBOH/ dpH
= - dCHA / dpH
A buffer solution is 0.20 M in acetic acid and
in sodium acetate. Calculate the change in pH
upon adding 1.0 mL of 0.10M hydrochloric acid
to 10 mL of this solution.
mmol HOAc = 2.0 + 0.1 = 2.1 mmol
mmol OAcˉ = 2.0 – 0.1 = 1.9 mmol
The change in pH is -0.05.
Since pOH =pKw -pH, we can also write , the above equation form pKw
Calculate the volume of concentrated ammonia
and the weight of ammonium chloride you
would have to take to prepare 100 mL of a
buffer at pH 10.00 if the final concentration of
salt is said to be 0.200 M
We want 100 mL 0f 0.200 M NH4Cl.
mmol NH4Cl = 0.200 mmol/mL × 100 mL
= 20.0 mmol
mg NH4Cl = 20.0 mmol × 53.5 mg/mmol
= 1.07 × 10³ mg
So, 1.07 g NH4Cl. Calculate [ ] of NH3 by
The molarity of concentrated ammonia is 14.8 M