CHEM504: CHAP7-ACID-BASE-EQUILLIBRIA

Who first recognized nitrogen as an essential plant nutrient and is hence often called the father of the fertilizer industry?

• Alessandro Volta

• Antoine-Laurent Lavoisier

• Robert Boyle

• Justus Von Liebig

Answer explanation

Liebig first recognized nitrogen as an essential plant nutrient and is hence often called the father of the fertilizer industry. Liebig first advocated experimentally centered teaching of chemistry. He was also the first to attempt to systematize organic chemistry.

Which of the following expressions most closely approximates the equilibrium constant of pure water?

• K_w = [H₂O][OH⁻]
  

• K_w = [H⁺][OH⁻]

• K_w = H⁺

Answer explanation

As the value of K_a increases, __________.

• the strength of the acid increases

• the strength of the conjugate base decreases

• the pKa decreases

• all of these

Answer explanation

K_a represents the equilibrium constant for the acid dissociation in water, 

HA → H⁺ + A^–, so it is a measure of the products divided by the reactants. As this value increases, the reaction favors the products (the ions) more, meaning that the acid dissociates more.

What is the pK_a of acetic acid? (K_a = 1.8 x 10^–5)

• 4.2

• 4.7

• 4.4

• 4.6

Answer explanation

We know that pK_a is equal to

–log(K_a). Thus, pK_a of acetic acid is –log(1.8 x 10^–5). The answer is 4.7.

Hypochlorous acid dissociates in water to create hydronium ions and hypochlorite ions HOCl + H₂O ⇌ H₃O⁺ + OCl^-. Suppose taht additional hypochlorite ions are added to the solution. Which of the following correctly describes the resultant effect of the concentration of HOCl?

• The concentration increases.

• The concentration decreases.

• The concentration remains the same.

• The concentration depends on the number of hydronium ions.

Answer explanation

Le Châtelier's principle states that equilibrated solutions tend to resist changes in the relative concentrations of chemical species.

The reaction thus seeks to minimize the effect of the added OCl⁻ ions, start superscript, minus, end superscript ions, tilting the equilibrium to the left.

The concentration of HOCl increases.

Which of the following Ka values represents the acid with the strongest conjugate base?

• 7.8 x 10^-2

• 4.2 x 10^-12

• 4.0 x 10^-9

• 7.5 x 10^-2

Which theory recognizes the ionization of a solvent to give a cation

and an anion?

• Arrhenius theory

• Brønsted–Lowry theory

• Lewis theory

• Theory of Solvent Systems

Which of the following 0.10M solutions will have the "highest" electrical conductivity?

• HIO₃

• H₃PO₄

• CH₃COO

• H₂S

Calculate the pOH and the pH of a 5.0 × 10⁻² M solution of NaOH at 25^◦C.

• 12

• 12.50

• 12.60

• 12.70

Answer explanation

pH = −log(2.0 × 10⁻¹³) = 13

− log 2.0 = 13 − 0.30 = 12.70

Students in a chemistry class are given one of four unknown samples in a laboratory. A student is told that his compound is the strongest acid of the four compounds. Based on the information in the above table, which compound was the student given?

• Compound 1

• Compound 2

• Compound 3

• Compound 4

Answer explanation

The strongest acid of the group will also have the smallest K_b value. As the weakest base (smallest K_b), compound 4 will only partially dissociate in solution because it has a fairly strong conjugate acid. Were this question asking which base was the strongest, compound 1 would be the answer, due to its large K_b value.