Free Acids, Bases and Salts MCQs with Answers

211. The most convenient and has nearest approach to a universal pH measurement is

• A. pH strips
• B. pH indicator
• C. The emf method
• D. The colorimetric method

Explanation: The emf method, often utilizing a pH meter, measures the voltage difference between electrodes and converts it to a pH value. This method provides precise, quantitative pH measurements across a wide range, making it the most universal and convenient option. In contrast, pH strips and indicators offer only rough estimates and are limited to specific pH ranges. The colorimetric method is also less precise and not ideal for universal applications.

212. Buffer solution are used to

• A. Increase the pH
• B. Decrease the pH
• C. Resist changes in pH
• D. None of the above

Explanation: Buffer solutions are used to resist changes in pH when small amounts of acid or base are added. They do this through a chemical equilibrium between a weak acid and its conjugate base (or a weak base and its conjugate acid). The correct answer, therefore, is to 'Resist changes in pH', as buffer solutions do not inherently increase or decrease pH, but rather maintain it within a narrow range. Options A and B are incorrect because buffers do not serve to increase or decrease pH outside of small adjustments to maintain equilibrium. Option D is incorrect because option C correctly describes the function of a buffer solution.

213. A mixture of weak acid and its salt is

• A. Alkaline buffer
• B. Acidic buffer
• C. Neutral buffer
• D. All of the above

Explanation: An acidic buffer is a solution that consists of a weak acid and its salt, which helps to maintain a stable pH level when small amounts of acid or base are added. This is the correct answer because the mixture described in the question specifically forms an acidic buffer. An alkaline buffer, on the other hand, consists of a weak base and its salt, not a weak acid. A neutral buffer is not typically created from a weak acid and its salt, as these combinations generally result in an acidic buffer. Therefore, option D 'All of the above' is incorrect because only the acidic buffer option accurately describes the given mixture.

214. Glass electrode cannot be used to measure the pH of pure

• A. Acetic acid
• B. Ethyl alcohol
• C. Gelatin
• D. All of the above

Explanation: A glass electrode is designed to measure pH in aqueous solutions with sufficient ionic strength. Pure acetic acid lacks the necessary conductivity without water, ethyl alcohol is non-aqueous and unable to support ion exchange, and gelatin's semi-solid nature and low ion mobility lead to unstable pH readings. Therefore, All of the above is the correct answer, as none of these substances provide the necessary conditions for accurate pH measurement using a glass electrode.

215. The pH value 4.2 is of

• A. Vinegar
• B. Lemons
• C. Orange
• D. Tomatoes

Explanation: The correct answer is Tomatoes. Tomatoes usually have a pH range from 4.0 to 4.6, which matches the given pH of 4.2. The other options are more acidic: Vinegar has a pH of about 2.5 to 3.5, Lemons have a pH of about 2 to 3, and Oranges have a pH of about 3 to 4. These values are all lower than 4.2, indicating they are more acidic than tomatoes.

216. If the pH of solution is 1, its pOH will

• A. 13
• B. 11
• C. 9
• D. None of the above

Explanation: The pOH of a solution can be calculated using the relationship: pH + pOH = 14. This equation holds true for aqueous solutions at 25°C. Given that the pH of the solution is 1, you can find the pOH by rearranging the equation to pOH = 14 - pH. Substituting the given pH value, pOH = 14 - 1 = 13. Therefore, the correct answer is Option A.Other options are incorrect because they do not satisfy the fundamental relationship between pH and pOH. Specifically, Option B suggests a pOH of 11, which would imply a pH of 3, and Option C suggests a pOH of 9, implying a pH of 5. Neither is consistent with the given pH of 1. Option D is incorrect as it asserts that none of the options is correct, which is false since Option A is correct.

217. Which of the following is not Bronsted acid?

• A. HCl
• B. H2O
• C. HS-
• D. Cl-

Explanation: The correct answer is Cl-. A Bronsted acid is defined as a substance that can donate a proton (H+). Cl- is the conjugate base of the strong acid HCl and lacks a proton to donate, thus it cannot act as a Bronsted acid. In contrast, HCl is a classic example of a Bronsted acid as it readily donates a proton. H2O, although neutral, can act as a Bronsted acid in some reactions by donating a proton to become OH-. Similarly, HS- can donate a proton to form S2-, playing the role of a Bronsted acid in some contexts.

218. Which of the following is not Lowery-Bronsted Base?

• A. NH3
• B. H2O
• C. HS-
• D. Cl-

Explanation: In the Brønsted-Lowry theory, a base is a species that can accept a proton (H+). NH3 and H2O can accept protons, forming NH4+ and H3O+, respectively. HS- can also accept a proton to form H2S. However, Cl-, being the conjugate base of a strong acid (HCl), has very low affinity for protons and does not generally act as a base in aqueous solutions. Therefore, Cl- is not a Brønsted-Lowry base.

219. Which of the following is not Lewis acid?

• A. HCl
• B. BF3
• C. AlCl3
• D. Cl-

Explanation: The correct answer is Cl-. A Lewis acid is defined as a substance that can accept an electron pair. In this context, Cl- is not a Lewis acid because it already possesses a full octet and is negatively charged, indicating it is more likely to donate an electron pair, thus acting as a Lewis base. On the other hand, HCl can become a Lewis acid when it dissociates, BF3 is electron-deficient and can accept electron pairs, and AlCl3 can also accept electron pairs due to its electron-deficient aluminum center.

220. Which of the following is not Lewis base?

• A. NH3
• B. H2O
• C. Pyridine
• D. BF3

Explanation: The correct answer is BF3. A Lewis base is a substance that can donate a pair of electrons. NH3, H2O, and pyridine all have lone pairs of electrons available for donation, making them Lewis bases. In contrast, BF3 lacks lone pairs of electrons and features an electron-deficient boron atom that can accept electron pairs, classifying it as a Lewis acid. Therefore, BF3 is not a Lewis base.