“Calculate the compositions of two buffers. Each buffer is to be 0.10 M ethanolamine (H2NCH2CH2OH) and...
“Calculate the compositions of two buffers. Each buffer is to be 0.10 M ethanolamine (H2NCH2CH2OH) and to contain acetic acid. One buffer is to be pH 5.0, the other 9.2. Which of these two buffers will have the greater buffering capacity?” Note: assume a volume of one liter for each buffer. The pKa’s of ethanolammonium ion (the protonated form of ethanolamine) and acetic acid are 9.44 and 4.76, respectively.
Homework Answers
A buffer is any type of substance that will resist pH change when H+ or OH- is added.
This is typically achieved with equilibrium equations. Both type of buffer will resist both type of additions.
When a weak acid and its conjugate base are added, they will form a buffer
The equations:
The Weak acid equilibrium:
HA(aq) <-> H+(aq) + A-(aq)
Weak acid = HA(aq)
Conjugate base = A-(aq)
Neutralization of H+ ions:
A-(aq) + H+(aq) <-> HA(aq); in this case, HA is formed, H+ is neutralized as well as A-, the conjugate
Neutralization of OH- ions:
HA(aq) + OH-(aq) <-> H2O(l) + A-(aq) ; in this case; A- is formed, OH- is neutralized as well as HA.
Now,
For the weak base equilibrium:
B(aq) + H2O(l) <-> BH+(aq) + OH-(aq)
Weak base = B;
Conjugate acid = BH+
Neutralization of OH- ions:
BH+(aq) + OH-(aq) <-> B(aq) + H2O(l); in this case, OH- is neutralized by BH+, as well as B is created
Neutralization of H+ ions:
B(aq) + H+(aq) <-> BH+(aq)
for
A) ethanolamine ion
+H3NCH2CH2OH <--> HNCH2CH2OH + H+
pH = pKa + log(HNCH2CH2OH / +H3NCH2CH2OH )
assume this is 9.2
9.2 = 9.44 + log(HNCH2CH2OH / +H3NCH2CH2OH )
ratio:
(HNCH2CH2OH / +H3NCH2CH2OH ) = 10^(9.20-9.44)
(HNCH2CH2OH / +H3NCH2CH2OH ) = 0.57543
b)
For acetic acid
CH3COOH <--> CH3COO- + H+
pH= pKa + log(CH3COO-/CH3COOH)
5.0= 4.76 + log(CH3COO-/CH3COOH)
(CH3COO-/CH3COOH) = 10^(5-4.76)
(CH3COO-/CH3COOH)= 1.73780
now, compare rations:
(HNCH2CH2OH / +H3NCH2CH2OH ) = 0.57543
(CH3COO-/CH3COOH)= 1.73780
if both species have 0.1 M, then...
we can only compare ratios
the ratio nearest to 1 will have the greatest "buffering capacity"
(HNCH2CH2OH / +H3NCH2CH2OH ) = 0.57543
(CH3COO-/CH3COOH)= 1.73780
Cleraly, the ethanolamine buffer is nearest to ratio 1, so it has higher buffer capacity... specially for Base addition
the acetate buffer is not that near to raito 1, but will have a good buffer capacity towards acid addition
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