Free Acids, Bases and Salts MCQs with Answers

231. A solution with pKa = -7, is

• A. Strong base
• B. Strong acid
• C. Neutral
• D. Weak acid

Explanation: The pKa value is an indicator of the strength of an acid. The lower the pKa value, the stronger the acid. A pKa of -7 is extremely low, indicating that the acid is very strong. Strong acids completely dissociate in water, which is consistent with a low or negative pKa value. Conversely, strong bases would be associated with high pKb values, and a neutral solution is not described by pKa. Weak acids have higher pKa values, generally above 0.

232. This relation Ka × 1/Kb shows that

• A. Weak acid have weak conjugate base
• B. Strong acid have strong conjugate base
• C. Strong acid have weak conjugate base
• D. Strong base have strong conjugate acid

Explanation: The relation Ka × 1/Kb is derived from the ionization constants of acids and bases. For a strong acid, the acid dissociation constant (Ka) is large, indicating that the acid dissociates completely in solution. Its conjugate base, therefore, has a low tendency to accept protons, making it weak. Thus, a strong acid has a weak conjugate base. Option A is incorrect because a weak acid typically has a stronger conjugate base. Option B is incorrect because a strong acid does not have a strong conjugate base. Option D is incorrect because a strong base has a weak conjugate acid.

233. Which of the following shows the limit of Ka value for strong acid?

• A. Ka < 10-3
• B. Ka = 1 to 10-3
• C. Ka > 1
• D. Ka = 0

Explanation: The strength of an acid is determined by its ability to donate protons, which is indicated by its acid dissociation constant, Ka. A strong acid dissociates almost completely in solution, resulting in a high Ka value. Therefore, strong acids have Ka values greater than 1. Option C is correct as it correctly represents the Ka value for strong acids. Option A and B suggest Ka values that are too low for strong acids, indicating weaker acids. Option D is not feasible for an acid, as a Ka of 0 would mean no dissociation occurs.

234. Which one is incorrect for one molar H2SO4 aqueous solution?

• A. [H+] > 10-7
• B. pH < 7
• C. [H+] = [OH-]
• D. All are correct

Explanation: The correct answer is Option C: [H+] = [OH-. This is incorrect because in a strong acid solution like 1 M H2SO4, the concentration of hydrogen ions is significantly higher than that of hydroxide ions, hence [H+] > [OH-]. Option A is correct because the concentration of H+ in a 1 M sulfuric acid solution is indeed greater than 10-7. Option B is correct because strong acids have a pH less than 7. Option D is incorrect as not all the given statements are correct.

235. pKa values of four solutions are given below, which one is more acidic?

• A. 10-3
• B. 10-4
• C. 250
• D. 10-2

Explanation: The acidity of a solution is inversely related to its pKa value; the lower the pKa, the stronger the acid. Therefore, among the given options, the solution with a pKa of 10-4 is the most acidic. Options with pKa values of 10-3 and 10-2 are less acidic, and a pKa of 250 indicates a very weak acid.

236. If pKa value of HCN is 3.7 then pKa value of CN is

• A. 7.3
• B. 3.7
• C. 10.3
• D. 14

Explanation: To find the pKb of CN-, which is the conjugate base of HCN, use the formula: pKa + pKb = 14. Given the pKa of HCN is 3.7, the calculation is: pKb = 14 - 3.7 = 10.3. Therefore, the correct answer is Option C: 10.3. Option A: 7.3 ignores this relationship. Option B: 3.7 mistakenly gives the pKa of HCN instead of the pKb of CN-. Option D: 14 is the sum of pKa and pKb, not the pKb value itself.

237. Very large value of pKa means the solution is

• A. Strongly basic
• B. Weakly acidic
• C. Basic
• D. Acidic

Explanation: The pKa value is a critical measure of an acid's strength; it is inversely proportional to the acid's ability to donate protons. A very large pKa value signifies a weak acid, meaning it donates protons inefficiently, leading to a solution that is weakly acidic. This is why Option B is correct. Options A and C mistakenly equate a weak acid with a basic solution, which is incorrect unless strong bases are present. Option D is incorrect because a large pKa denotes weak, not strong, acidity.

238. The correct relationship between Ka & Kb at 25°C for water is

• A. Ka × Kb = Kw
• B. Ka × Kb = [H+][OH-]
• C. Ka × Kb = 10-14
• D. None of these

Explanation: The correct relationship between Ka and Kb at 25°C is Ka × Kb = Kw, where Kw is the ion product of water, equal to 1.0 × 10-14 at this temperature. Option A is correct as it clearly states this relationship. Option B, while equivalent to Kw's definition, does not explicitly mention it, making it less clear. Option C provides the numerical value of Kw but omits the term, reducing clarity. Option D is incorrect because Option A accurately expresses the relationship.

239. Which of the following relationship is correct?

• A. Ka = 1/Kb
• B. Ka = Kb
• C. pKw = log Kw
• D. None of these

Explanation: The correct answer is 'None of these' because none of the provided relationships reflect the true mathematical expressions involving dissociation constants. The accurate relationship for a conjugate acid-base pair is Kw = Ka * Kb. Additionally, pKw should be expressed as -log Kw, not log Kw. Thus, options A, B, and C are incorrect.

240. Lower the pKa value means

• A. Strong acidic solution
• B. Lower pH of solution
• C. [OH-] > [H+]
• D. Higher Ka value

Explanation: The pKa value is the negative logarithm of the acid dissociation constant (Ka). A lower pKa value indicates a higher Ka value, suggesting that the acid dissociates more completely in solution, thus it is a stronger acid. Therefore, Option D is correct. Option A is incorrect because a strong acidic solution relates to low pH, not directly to pKa. Option B is incorrect as lower pKa does not always mean lower pH without considering the acid concentration. Option C is incorrect because a lower pKa suggests more H+ ions, not OH- ions.