Acids, Bases, and Solution Equilibria – Guided Practice

Q1. Which one of the following will form an acidic solution in water?

Background

Topic: Acid-Base Properties of Salts

This question tests your understanding of how salts affect the pH of aqueous solutions, specifically which ions hydrolyze to produce acidic or basic solutions.

Key Terms and Concepts:

• Acidic solution: A solution with pH < 7, containing excess H3O+.

• Salt hydrolysis: The reaction of salt ions with water to produce H3O+ or OH-.

• Conjugate acid/base: The species formed when an acid donates or a base accepts a proton.

Step-by-Step Guidance

1. Identify the ions produced when each salt dissolves in water (e.g., NH4Cl → NH4+ + Cl-).

2. Determine whether each ion is the conjugate acid or base of a strong or weak acid/base.

3. Recall that the conjugate acid of a weak base will hydrolyze to produce H3O+ (acidic solution).

4. Analyze each option to see which salt contains an ion that will make the solution acidic.

Try solving on your own before revealing the answer!

Q2. The acid-dissociation constant of hydrocyanic acid (HCN) at 25.0°C is 4.9 × 10-10. What is the pH of an aqueous solution of 0.080 M sodium cyanide (NaCN)?

Background

Topic: Weak Base Equilibria and Salt Solutions

This question tests your ability to calculate the pH of a solution containing the salt of a weak acid (NaCN), using the relationship between Ka and Kb.

Key Terms and Formulas:

• Acid dissociation constant ($K_a$): $K_a$ for HCN is given.

• Base dissociation constant ($K_b$): $K_b$ for CN- can be found using $K_w = K_a \times K_b$.

• pH and pOH: $pH + pOH = 14$ at 25°C.

Step-by-Step Guidance

1. Write the hydrolysis equation for CN-: $\mathrm{CN^- + H_2O \rightleftharpoons HCN + OH^-}$.

2. Calculate $K_b$ for CN- using $K_w = K_a \times K_b$ ($K_w = 1.0 \times 10^{-14}$ at 25°C).

3. Set up an ICE table for the hydrolysis reaction, using the initial concentration of CN- (0.080 M).

4. Write the expression for $K_b$ and solve for $[OH^-]$ at equilibrium.

5. Calculate pOH from $[OH^-]$, then use $pH + pOH = 14$ to find the pH.

Try solving on your own before revealing the answer!

Q3. Determine the ammonia concentration of an aqueous solution that has a pH of 11.00. The equation for the dissociation of NH3 (Kb = 1.8 × 10-5) is:

NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH-(aq)

Background

Topic: Weak Base Equilibria

This question tests your ability to relate pH, pOH, and base concentration using equilibrium concepts for weak bases.

Key Terms and Formulas:

• $K_b$ (base dissociation constant): $K_b = 1.8 \times 10^{-5}$

• pH and pOH: $pH + pOH = 14$

• $[OH^-] = 10^{-pOH}$

• ICE table for equilibrium calculations

Step-by-Step Guidance

1. Calculate pOH using $pOH = 14 - pH$.

2. Find $[OH^-]$ using $[OH^-] = 10^{-pOH}$.

3. Set up an ICE table for the dissociation of NH3 and express all concentrations in terms of $x$ (change in $[OH^-]$).

4. Write the $K_b$ expression and substitute the equilibrium values.

Try solving on your own before revealing the answer!

Q4. What is the pH of a 0.100 M NH3 solution that has Kb = 1.8 × 10-5?

NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH-(aq)

Background

Topic: Weak Base Equilibria

This question tests your ability to calculate the pH of a weak base solution using equilibrium concepts and $K_b$.

Key Terms and Formulas:

• $K_b$ (base dissociation constant): $K_b = 1.8 \times 10^{-5}$

• ICE table for equilibrium calculations

• pOH and pH relationship: $pH + pOH = 14$

Step-by-Step Guidance

1. Set up an ICE table for the dissociation of NH3 with an initial concentration of 0.100 M.

2. Write the $K_b$ expression and substitute the equilibrium values.

3. Solve for $[OH^-]$ at equilibrium.

4. Calculate pOH from $[OH^-]$, then use $pH + pOH = 14$ to find the pH.

Try solving on your own before revealing the answer!

Q5. Calculate the pOH of a nitrous acid solution that contains 3.9 × 10-8 M H3O+ at 25°C.

Background

Topic: pH and pOH Calculations

This question tests your ability to relate hydronium ion concentration to pOH using the properties of water at 25°C.

Key Terms and Formulas:

• $[H_3O^+]$ (hydronium ion concentration)

• $pH = -\log([H_3O^+])$

• $pH + pOH = 14$ at 25°C

Step-by-Step Guidance

1. Calculate pH using $pH = -\log([H_3O^+])$.

2. Use $pH + pOH = 14$ to solve for pOH.

Try solving on your own before revealing the answer!

Q6. Identify the strongest acid.

Background

Topic: Oxoacid Strength Trends

This question tests your understanding of periodic trends and structure in determining acid strength, especially for oxoacids of the same central atom.

Key Terms and Concepts:

• Oxoacid: An acid with hydrogen, oxygen, and another element (the central atom).

• Acid strength increases with more oxygen atoms attached to the central atom.

Step-by-Step Guidance

1. List the formulas for each acid and count the number of oxygen atoms.

2. Recall that for oxoacids with the same central atom, acid strength increases with more oxygens.

3. Identify which acid has the most oxygen atoms.

Try solving on your own before revealing the answer!