Final Exam: Chem 112

Q1. Of the following, which is the strongest acid?

Background

Topic: Acid Strength and Oxyacids

This question tests your understanding of trends in acid strength, especially among oxyacids (acids containing oxygen), and how the structure and electronegativity of the central atom affect acidity.

Key Terms and Concepts:

• Oxyacid: An acid in which the acidic proton is bonded to an oxygen atom.

• Acid Strength: The tendency of an acid to donate a proton (H+). Stronger acids dissociate more completely in water.

• Trends: For oxyacids with the same central atom, more oxygens = stronger acid. For acids with the same number of oxygens, higher electronegativity of the central atom = stronger acid.

Step-by-Step Guidance

1. Identify the central atom and the number of oxygen atoms in each acid (HClO, HIO, HClO2, HClO4, HClO3).

2. Recall that for oxyacids with the same central atom, acid strength increases with the number of oxygens attached.

3. For acids with the same number of oxygens, compare the electronegativity of the central atom (Cl vs. I).

4. Arrange the acids in order of increasing acid strength based on these trends.

Try solving on your own before revealing the answer!

Final Answer: D) HClO4

Perchloric acid (HClO4) has the most oxygens and the most electronegative central atom, making it the strongest acid among the choices.

Q2. Of the compounds below, a 0.1 M aqueous solution of _______ will have the highest pH.

Background

Topic: Acid-Base Chemistry, Salts in Water

This question tests your ability to predict the pH of salt solutions, considering whether the salt is derived from a strong/weak acid or base.

Key Terms and Concepts:

• pH: A measure of the hydrogen ion concentration; higher pH = more basic solution.

• Salt Hydrolysis: Some salts react with water to produce acidic or basic solutions depending on their parent acid/base.

• Weak Acid/Weak Base: Salts from weak acids or bases can hydrolyze to affect pH.

Step-by-Step Guidance

1. For each salt, identify if it comes from a strong or weak acid/base.

2. Recall that salts from weak acids and strong bases (like sodium acetate) will make the solution basic (pH > 7).

3. Compare the basicity of the anions (using Kb or Ka values provided) to determine which will produce the highest pH.

4. Set up the hydrolysis equation for the most likely candidate and consider the relative strengths.

Try solving on your own before revealing the answer!

Final Answer: B) KCN

KCN is the salt of a weak acid (HCN) and a strong base (KOH), so it hydrolyzes to give a basic solution with the highest pH among the options.

Q3. Which of the following aqueous solutions has the highest [OH-]?

Background

Topic: pH, pOH, and Ion Concentrations

This question tests your understanding of the relationship between pH, pOH, and the concentration of hydroxide ions in solution.

Key Terms and Formulas:

• pOH: $\mathrm{pOH} = -\log[\mathrm{OH}^-]$

• pH + pOH = 14 (at 25°C)

• [OH-]: $[\mathrm{OH}^-] = 10^{-\mathrm{pOH}}$

Step-by-Step Guidance

1. For each option, determine the [OH-] either directly (if pOH is given) or by converting from pH or concentration.

2. For strong acids/bases, assume complete dissociation to find [OH-] or [H+], then use $[\mathrm{OH}^-] = \frac{K_w}{[\mathrm{H}^+]}$ if needed.

3. Compare the calculated [OH-] values for each solution.

4. Identify which solution has the highest [OH-].

Try solving on your own before revealing the answer!

Final Answer: E) pure water

Pure water at 25°C has [OH-] = 1.0 × 10-7 M, which is higher than the [OH-] in the other acidic solutions listed.

Q4. The Ka for formic acid (HCO2H) is 1.8 × 10-4. What is the pH of a 0.10 M aqueous solution of sodium formate (NaHCO2)?

Background

Topic: Weak Acid/Base Equilibria, Hydrolysis of Salts

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

Key Terms and Formulas:

• Ka: Acid dissociation constant

• Kb: Base dissociation constant, $K_b = \frac{K_w}{K_a}$

• Hydrolysis Equation: $\mathrm{HCO}_2^- + \mathrm{H}_2O \rightleftharpoons \mathrm{HCO}_2H + \mathrm{OH}^-$

• pOH and pH: $\mathrm{pOH} = -\log[\mathrm{OH}^-]$, $\mathrm{pH} = 14 - \mathrm{pOH}$

Step-by-Step Guidance

1. Calculate $K_b$ for the formate ion using $K_b = \frac{K_w}{K_a}$, where $K_w = 1.0 \times 10^{-14}$.

2. Set up the equilibrium expression for the hydrolysis of formate: $K_b = \frac{[\mathrm{HCO}_2H][\mathrm{OH}^-]}{[\mathrm{HCO}_2^-]}$.

3. Let $x$ be the amount of $[\mathrm{OH}^-]$ formed. Substitute the initial concentration and solve for $x$ using the $K_b$ expression.

4. Once you have $[\mathrm{OH}^-]$, calculate pOH, then use $\mathrm{pH} = 14 - \mathrm{pOH}$.

Try solving on your own before revealing the answer!

Final Answer: D) 8.37

The solution is basic because sodium formate is the salt of a weak acid and a strong base. Calculating as above gives a pH of 8.37.

Q5. Which one of the following pairs cannot be mixed together to form a buffer solution?

Background

Topic: Buffer Solutions

This question tests your understanding of what constitutes a buffer: a mixture of a weak acid and its conjugate base, or a weak base and its conjugate acid.

Key Terms and Concepts:

• Buffer: A solution that resists changes in pH when small amounts of acid or base are added.

• Buffer Pair: Must contain a weak acid and its conjugate base, or a weak base and its conjugate acid.

Step-by-Step Guidance

1. For each pair, identify if they are a weak acid/base and its conjugate, or if one is a strong acid/base.

2. Recall that mixing a strong acid with a salt of a weak acid does not form a buffer.

3. Eliminate pairs that do not fit the buffer definition.

4. Identify the pair that cannot form a buffer.

Try solving on your own before revealing the answer!

Final Answer: B) KOH, HI

KOH is a strong base and HI is a strong acid; mixing them does not produce a buffer.