Test on Strong and weak acids

Question 1: For a strong acid dissolved in water, only a small fraction of the acid molecules donate a proton.

A. Yes

B. No

Explanation: Strong acids completely dissociate in water, meaning virtually all of the acid molecules donate a proton. This is in contrast to weak acids, where only a small fraction of molecules dissociate.

Question 2: Using the approximation method described in the study materials, what is the calculated value of 'x' (representing [H⁺]) for a 0.100 M solution of a weak acid if its Ka is 1.8 × 10⁻⁵?

A. x ≈ 1.34 × 10⁻³ M

B. x ≈ 1.8 × 10⁻⁴ M

C. x ≈ 1.00 × 10⁻⁷ M

D. x ≈ 4.24 × 10⁻³ M

Explanation: According to the study materials, for a weak acid HA, the Ka expression is Ka = [H⁺][A⁻] / [HA]. Using the ICE table, if the initial concentration of HA is 0.100 M, and 'x' represents the change in concentration due to dissociation, then at equilibrium [HA] = 0.100 - x, [H⁺] = x, and [A⁻] = x. The equation becomes Ka = (x)(x) / (0.100 - x). The study materials state that we can often assume 'x' is much smaller than the initial concentration, simplifying the equation to x² / 0.100 ≈ Ka. Plugging in the given Ka = 1.8 × 10⁻⁵: x² / 0.100 ≈ 1.8 × 10⁻⁵. This leads to x² ≈ 1.8 × 10⁻⁵ × 0.100, so x² ≈ 1.8 × 10⁻⁶. Taking the square root of 1.8 × 10⁻⁶ gives x ≈ 0.00134 M, or 1.34 × 10⁻³ M.

Question 3: The pH of a weak acid solution is typically lower than the pH of a strong acid solution of the same concentration, as indicated by the example provided.

A. Yes

B. No

Explanation: The study materials state that the pH of the weak acid (2.87) is significantly higher than the pH of a 0.100 M strong acid (1.00), illustrating that weak acids have a higher pH (meaning they are less acidic) compared to strong acids of the same concentration due to partial dissociation.

Question 4: The pH of 2.87 was calculated based on an [H⁺] concentration of 0.100 M.

A. Yes

B. No

Explanation: The pH of 2.87 was calculated from an [H⁺] concentration of 0.00134 M, as shown by pH = -log₁₀(0.00134) ≈ 2.87. A 0.100 M strong acid would have a pH of 1.00.

Question 5: Based on the provided study materials, what percentage value is used to determine if an approximation made during a weak acid calculation is considered valid?

A. 1.34%

B. 5%

C. 0.00134%

D. 100%

Explanation: The study materials state: '(0.00134 / 0.100) × 100% = 1.34%, which is less than 5%, so the approximation is valid.' This clearly indicates that the threshold for a valid approximation is being less than 5%.