Physical Chemistry - Basic Concepts
Q. If the vapor pressure of pure solvent is 100 mmHg, and the vapor pressure of the solution is 80 mmHg, what is the mole fraction of the solute? (2022)
A.
0.2
B.
0.4
C.
0.5
D.
0.8
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Solution
Using Raoult's Law: P_solution = X_solvent * P_solvent. X_solvent = P_solution / P_solvent = 80/100 = 0.8, thus X_solute = 1 - X_solvent = 0.2.
Correct Answer: B — 0.4
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Q. In a chemical reaction, what does the term 'activation energy' refer to? (2023)
A.
Energy released during the reaction
B.
Energy required to initiate the reaction
C.
Energy needed to maintain the reaction
D.
Energy that is lost
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Solution
Activation energy is the minimum energy required to initiate a chemical reaction.
Correct Answer: B — Energy required to initiate the reaction
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Q. What is the formula for calculating the number of moles from mass and molar mass? (2023)
A.
moles = mass × molar mass
B.
moles = mass / molar mass
C.
moles = molar mass / mass
D.
moles = mass + molar mass
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Solution
The correct formula is moles = mass / molar mass.
Correct Answer: B — moles = mass / molar mass
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Q. What is the pH of a solution with a hydrogen ion concentration of 1 x 10^-3 M? (2019)
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Solution
pH = -log[H+] = -log(1 x 10^-3) = 3.
Correct Answer: B — 3
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Q. Which of the following statements is true regarding exothermic reactions? (2021)
A.
They absorb heat.
B.
They release heat.
C.
They require energy input.
D.
They have a positive enthalpy change.
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Solution
Exothermic reactions release heat, resulting in a negative enthalpy change.
Correct Answer: B — They release heat.
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