chemistry : class11 pH of Common Ion Solutions : Common ion effect : Equilibrium MCQ Questions and Answers
Practice important
pH of Common Ion Solutions
MCQ questions and answers.
Page 1
of
1
|
10
MCQ Questions
1. A solution contains 0.100 mol L−1 acetic acid and 0.200 mol L−1 sodium acetate. Given Ka for acetic acid = 1.8 × 10−5, what is the approximate pH of the solution?
2. A solution is prepared by mixing 0.050 mol L−1 NH3 and 0.100 mol L−1 NH4Cl. If Kb for NH3 is 1.8 × 10−5, what is the approximate pH of the resulting common-ion solution?
3. What is the approximate pH of a solution containing equal concentrations, 0.100 mol L−1 each, of acetic acid and sodium acetate? Take Ka for acetic acid as 1.8 × 10−5.
A.
3.74
B.
4.26
C.
4.74
D.
5.26
Correct Answer:
4.74
Explanation:
Because the concentrations of acetic acid and acetate ion are equal, [H+] = Ka. Hence pH = pKa = −log(1.8 × 10−5) ≈ 4.74.
4. A solution contains 0.200 mol L−1 formic acid and 0.050 mol L−1 sodium formate. If Ka for formic acid is 1.8 × 10−4, what is the approximate pH?
A.
3.14
B.
3.44
C.
3.74
D.
4.04
Correct Answer:
3.44
Explanation:
The common formate ion suppresses the ionization of formic acid. [H+] = Ka[HCOOH]/[HCOO−] = (1.8 × 10−4)(0.200/0.050) = 7.2 × 10−4 mol L−1. Therefore, pH ≈ 3.14, not 3.44. Rechecking: −log(7.2 × 10−4) = 3.14. Thus the correct answer is 3.14.
5. One hundred millilitres of 0.200 mol L−1 acetic acid is mixed with 200 mL of 0.100 mol L−1 sodium acetate. If Ka for acetic acid is 1.8 × 10−5, what is the approximate pH after mixing?
A.
4.26
B.
4.44
C.
4.74
D.
5.04
Correct Answer:
4.74
Explanation:
Moles of acetic acid = 0.200 × 0.100 = 0.020 mol, and moles of acetate ion = 0.100 × 0.200 = 0.020 mol. Their concentrations after mixing remain equal in ratio, so pH = pKa = −log(1.8 × 10−5) ≈ 4.74.
6. A solution contains 0.020 mol L−1 benzoic acid and 0.100 mol L−1 sodium benzoate. Given Ka for benzoic acid = 6.3 × 10−5, what is the approximate pH?
7. A buffer contains 0.100 mol L−1 acetic acid and 0.100 mol L−1 acetate ion. A small amount of HCl is added so that the concentrations become 0.150 mol L−1 acetic acid and 0.050 mol L−1 acetate ion. If Ka = 1.8 × 10−5, what is the approximate pH?
A.
4.26
B.
4.44
C.
4.74
D.
5.22
Correct Answer:
4.26
Explanation:
The added HCl converts acetate ion into acetic acid. Thus [H+] = Ka[CH3COOH]/[CH3COO−] = (1.8 × 10−5)(0.150/0.050) = 5.4 × 10−5 mol L−1. Hence pH ≈ 4.26.
8. A solution contains equal concentrations of NH3 and NH4Cl. If Kb for NH3 is 1.8 × 10−5 and Kw = 1.0 × 10−14, what is the approximate pH?
A.
8.75
B.
9.25
C.
9.75
D.
10.25
Correct Answer:
9.25
Explanation:
For equal concentrations of NH3 and NH4+, [OH−] = Kb = 1.8 × 10−5. Therefore pOH ≈ 4.75 and pH = 14.00 − 4.75 = 9.25. Equivalently, pH = pKa of NH4+.
9. A solution contains 0.100 mol L−1 acetic acid and 0.250 mol L−1 sodium acetate. Its pH is measured as 5.00. What is the value of Ka for acetic acid based on these data?
A.
2.5 × 10−6
B.
4.0 × 10−6
C.
2.5 × 10−5
D.
4.0 × 10−5
Correct Answer:
2.5 × 10−5
Explanation:
Since pH = 5.00, [H+] = 1.0 × 10−5 mol L−1. For a common-ion solution, Ka = [H+][acetate]/[acetic acid] = (1.0 × 10−5)(0.250/0.100) = 2.5 × 10−5.
10. Two solutions have the same ratio of weak acid to its conjugate-base salt, but one solution is ten times more dilute than the other. Assuming the concentrations remain sufficiently large for the common-ion approximation, how do their pH values compare?
A.
The more dilute solution has a pH lower by 1 unit.
B.
The more dilute solution has a pH higher by 1 unit.
C.
Both solutions have approximately the same pH.
D.
The pH difference depends only on the volume of water added.
Correct Answer:
Both solutions have approximately the same pH.
Explanation:
For a weak acid and its common conjugate-base ion, [H+] = Ka[acid]/[conjugate base]. If both concentrations are diluted by the same factor, their ratio remains unchanged, so the pH remains approximately constant.
Short Questions and Answers
Practice short questions and answers related to
pH of Common Ion Solutions
.
Short questions are currently unavailable for this page.