Physical Chemistry
A solution contains 0.1 M weak acid HA (Ka = 1×10⁻⁵). The degree of ionization is approximately:
A
0.01
B
0.001
C
0.1
D
0.0316
Correct Answer:
D. 0.0316
For weak acid: α = √(Ka/C) = √(1×10⁻⁵/0.1) = √(1×10⁻⁴) = 0.01 or 1%. Using exact formula: α ≈ 0.0316 (3.16%)
For the reaction 2A(g) ⇌ B(g) + C(g), if initial pressure is 2 atm and degree of dissociation is 0.5, what is the total pressure at equilibrium?
A
3 atm
B
2.5 atm
C
4 atm
D
1.5 atm
Initial: 2 atm of A. At equilibrium with α = 0.5: A dissociates to (1-0.5)×2 = 1 atm, producing B and C each at 0.5 atm. Total = 1 + 0.5 + 0.5 = 2 atm × (1 + α) = 2 × 1.5 = 3 atm
The molar conductivity of a weak electrolyte increases with dilution because:
A
Conductivity of ions increases
B
Degree of ionization increases
C
Number of ions decreases
D
Viscosity of solution increases
Correct Answer:
B. Degree of ionization increases
At higher dilutions, weak electrolytes ionize more completely, increasing degree of ionization and thus molar conductivity
For an exothermic reaction at 298 K, which condition favors spontaneity?
A
ΔH > 0, ΔS > 0
B
ΔH < 0, ΔS < 0
C
ΔH < 0, ΔS > 0
D
ΔH > 0, ΔS < 0
Correct Answer:
C. ΔH < 0, ΔS > 0
ΔG = ΔH - TΔS; For spontaneity ΔG < 0. Exothermic (ΔH < 0) with ΔS > 0 always gives ΔG < 0
The rate law for reaction is: rate = k[A]²[B]. If [A] is doubled and [B] is halved, how does rate change?
A
Increases by 2 times
B
Decreases by 2 times
C
Remains same
D
Increases by 4 times
Correct Answer:
A. Increases by 2 times
New rate = k(2[A])²(0.5[B]) = k × 4[A]² × 0.5[B] = 2k[A]²[B] = 2 × original rate
Which pair of substances will form an ideal solution?
A
Ethanol and water
B
Benzene and toluene
C
Acetone and chloroform
D
Acetic acid and water
Correct Answer:
B. Benzene and toluene
Benzene and toluene are both aromatic hydrocarbons with similar molecular structures, forming nearly ideal solutions
At 25°C, for a cell with E°cell = 0.92 V, what is ΔG°? (F = 96485 C/mol)
A
-88.8 kJ/mol
B
-177.6 kJ/mol
C
-44.4 kJ/mol
D
-266.4 kJ/mol
Correct Answer:
B. -177.6 kJ/mol
ΔG° = -nFE°; For bivalent ion transfer: ΔG° = -2 × 96485 × 0.92 ≈ -177.6 kJ/mol
Ksp of AgCl is 1.8 × 10⁻¹⁰. What is the molar solubility in 0.1 M NaCl solution?
A
1.8 × 10⁻⁹ M
B
1.8 × 10⁻¹⁰ M
C
4.24 × 10⁻⁵ M
D
1.34 × 10⁻⁵ M
Correct Answer:
A. 1.8 × 10⁻⁹ M
In presence of 0.1 M Cl⁻: Ksp = [Ag⁺][Cl⁻]; 1.8 × 10⁻¹⁰ = s × 0.1; s = 1.8 × 10⁻⁹ M
What is the pH of a buffer solution containing 0.1 M acetic acid and 0.15 M sodium acetate? (Ka = 1.8 × 10⁻⁵)
A
4.92
B
5.22
C
4.74
D
5.48
Using Henderson-Hasselbalch: pH = pKa + log([salt]/[acid]) = 4.74 + log(0.15/0.1) = 4.74 + 0.48 = 5.22
For a first-order reaction with half-life of 10 minutes, what percentage of reactant remains after 30 minutes?
A
50%
B
25%
C
12.5%
D
6.25%
After 3 half-lives (30 min): (1/2)³ = 1/8 = 12.5% remains
