Free Acids, Bases and Salts MCQs with Answers

71. The pKa value of CH3COOH is 4.74, the pH of equimolar solution of acetic acid and sodium acetate is:

• A. 13.0
• B. 7.2
• C. 4.79
• D. 4.74

Explanation: To determine the pH of an equimolar solution of acetic acid (CH3COOH) and sodium acetate (CH3COONa), we need to consider the acid-base equilibrium between acetic acid and its conjugate base acetate ion. The pKa value of acetic acid (CH3COOH) is given as 4.74. The pKa is a measure of the acidity of a compound and is defined as the negative logarithm of the acid dissociation constant (Ka). In this case, we have an equimolar solution of acetic acid and sodium acetate. Sodium acetate (CH3COONa) is the salt of the conjugate base acetate ion (CH3COO-) and a sodium cation (Na+). When the salt dissolves in water, it dissociates into acetate ions and sodium ions. In the equimolar solution, the concentration of acetic acid and acetate ions will be the same. Since acetic acid is a weak acid, it will partially dissociate into acetate ions and release H+ ions. The Henderson-Hasselbalch equation relates the pH of a solution to the pKa and the ratio of the concentrations of the conjugate acid and base: pH = pKa + log([A-]/[HA]) In this case, [A-] and [HA] represent the concentrations of the acetate ion and acetic acid, respectively. Since we have an equimolar solution, the concentration of acetate ions ([A-]) will be equal to the concentration of acetic acid ([HA]). pH = pKa + log(1) pH = pKa Therefore, the pH of the equimolar solution of acetic acid and sodium acetate will be equal to the pKa value of acetic acid, which is 4.74. Using Henderson's Equation: pH = pKa + log ([Salt]/[Acid]). where; Acid= Acetic Acid Salt= Sodium acetate [Salt]/[Acid]= 1 (as equimolar), hence log ([Salt]/[Acid])= 0 Therefore, pH= pKa= 4.74

72. In this reaction NH4+ + H2O -----> H3O+, the conjugate base of H3O+ ion is:

• A. NH4+
• B. H2O
• C. H+
• D. NH3

Explanation: In the Brønsted-Lowry definition of acids and bases, a conjugate acid-base pair consists of two substances that differ only by the presence of a proton (H⁺). A conjugate acid is formed when a proton is added to a base, and a conjugate base is formed when a proton is removed from an acid. In this reaction H2O is accepting an H+ to form H3O+, hence it is the conjugate base of H3O+.

73. The pH of 10-3 mol dm-3 of an aqueous solution H2SO4 is:

• A. 3.0
• B. 2.7
• C. 2.0
• D. 1.5

Explanation: Given pH= 10-3 After ionization, H2SO4 ⇔ 2H+ + SO4- [H+] from H2SO4 = 2x10-3 We know that pH = -log10 [H+] pH = -log10 (2 x 10-3) = 3 - log2 = 3 - 1.414 = 2.69M

74. The Ka values of HCI, CH3COOH, HF, and H2SO4 are 107, 1.85 x 10^-5, 6.7 x 10^-5 and 102 respectively. The decreasing order of acidic strength is:

• A. CH3COOH > HF > H2SO4 > HCI
• B. HCI > H2SO4 > HF > CH3COOH
• C. HCl > CH3COOH > HF > H2SO4
• D. HCl > HF > H2SO4 > CH3COOH

Explanation: Ka is a measure of ionization of the acid, thereby a greater Ka corresponds to greater acidic strength. The question asks for DECREASING order of acidic strength so, the acids are to be listed from strongest to weakest. HCl is the strongest with the greatest Ka (107), followed by H2SO4 (102), followed by HF (6.7 x 10-5), and then CH3COOH which is the weakest having the lowest Ka (1.85 x 10-5). Option B has this correctly laid out so, it is the answer. Option A has the order in reverse, as it lists the acid down in increasing order of acidic strength. Options, C and D, have them listed incorrectly, as C has CH3COOH before the rest while D has HF mentioned before H2SO4.

75. According to Lowry - Bronsted Acid & Base Concept, H2O is:

• A. An acid
• B. A base
• C. An amphoteric species
• D. A salt

Explanation: Water is an example of an amphiprotic substance. This means it is able to donate a proton or accept a proton. The Bronsted/Lowry theory defines an acid or base by how they behave in a chemical reaction. Depending on what water reacts with it can act as an acid (proton donor) or a base (proton acceptor).

76. Conjugate base of a weak acid:

• A. Weak
• B. Strong
• C. Unstable
• D. None of these options

Explanation: Weak acids have strong bonds to their H+. Strong acids have weak bonds to their H+. Weak bases have a strong tendency to get H+ and/or donate an electron pair. Strong bases have a weak tendency to get H+ and/or donate an electron pair. Hence conjugate base of weak acid is strong. So, B is the correct option.

77. Which of the following is a strong acid?

• A. Ethane
• B. Ethyl Chloride
• C. Ethanol
• D. Phenol

Explanation: Phenols are much more acidic than alcohols because the negative charge in the phenoxide ion is not localized on the oxygen atom, as it is in an alkoxide ion, but is delocalized-it is shared by a number of carbon atoms in the benzene ring. On ther other hand, ethane and ethyl chloride doesn't display acidic behavior. Thus, option D is correct.

78. What will be the pH of a solution of Mg(OH)2 having 10-³ mol / dm³ concentration?

• A. 11
• B. 6
• C. 3
• D. 10

Explanation: In order to calculate this pH, we assume that all of Mg(OH)2 dissociates in solution. As pH + pOH = 14, Calculating pOH first: pOH = -log[OH-] pOH = -log[10-3mol dm-3] pOH = 3 Now, pH + 3 = 14 pH = 14-3 pH = 11

79. Which of the following is true about buffers?

• A. pH is not affected by time
• B. All of these
• C. Have definite pH
• D. pH is not affected by dilution

Explanation: Buffers are solutions that resist the change in pH when a small amount of acid or base is added to them. They have a constant pH and their pH values do not change on dilution and with time.

80. After the hydrolysis of ester the change in the concentration of acid at different intervals is calculated by _:

• A. Titration with KMnO4
• B. Titration with standard alkali
• C. Distillation
• D. Evaporation of mixture

Explanation: The change in the concentration of acid at different intervals after the hydrolysis of ester is calculated by titration with standard alkali. In the hydrolysis of ester, the ester is broken down into an alcohol and a carboxylic acid. The carboxylic acid can be titrated with a standard alkali, such as sodium hydroxide (NaOH). The amount of alkali required to neutralize the acid is a measure of the concentration of the acid. The titration is carried out by adding the standard alkali to the acid solution until the acid is neutralized. The endpoint of the titration is indicated by a change in the pH of the solution.