General Chemistry Study Guidance: Equilibrium, Acids & Bases, and Solution Calculations

Q1. A typical aspirin tablet contains 325 mg of aspirin (C9H8O4). Aspirin is a monoprotic acid with $K_a = 3.0 \times 10^{-4}$. If you dissolve four tablets of aspirin in 440 mL of H2O, what is the pH?

Background

Topic: Acid-Base Equilibria

This question tests your ability to calculate the pH of a solution containing a weak acid, using its mass, molar mass, and acid dissociation constant ($K_a$).

Key Terms and Formulas

• Monoprotic acid: An acid that donates one proton per molecule.

• $K_a$: Acid dissociation constant.

• $pH = -\log[H^+]$

• For a weak acid: $K_a = \frac{[H^+][A^-]}{[HA]}$

Step-by-Step Guidance

1. Calculate the total mass of aspirin dissolved: $\text{Total mass} = 4 \times 325\,\text{mg}$.

2. Convert the mass to moles using the molar mass of aspirin ($C_9H_8O_4$): $\text{Moles} = \frac{\text{mass}}{\text{molar mass}}$.

3. Calculate the initial concentration of aspirin in the solution: $[HA]_0 = \frac{\text{moles}}{0.440\,\text{L}}$.

4. Set up the equilibrium expression for the dissociation of aspirin: $HA \rightleftharpoons H^+ + A^-$.

5. Write the $K_a$ expression and set up the ICE table to solve for $[H^+]$.

Try solving on your own before revealing the answer!

Q2. The pH of 0.040 M hydroferrous acid (HFO2H) is 5.05.

Background

Topic: Acid Dissociation and Strength

This question tests your ability to calculate the acid dissociation constant ($K_a$) from pH and concentration, and to determine whether the acid is strong or weak.

Key Terms and Formulas

• $pH = -\log[H^+]$

• $K_a = \frac{[H^+][A^-]}{[HA]}$

• Strong acid: Nearly complete dissociation; weak acid: partial dissociation.

Step-by-Step Guidance

1. Use the pH to find $[H^+]$: $[H^+] = 10^{-pH}$.

2. Assume $[A^-] \approx [H^+]$ and $[HA]_{eq} \approx [HA]_0 - [H^+]$.

3. Plug values into the $K_a$ expression: $K_a = \frac{[H^+][A^-]}{[HA]_{eq}}$.

4. Compare the calculated $K_a$ to typical values for strong and weak acids to answer part (b).

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Q3. Calculate the pH of the following solutions:

Background

Topic: Solution Chemistry and pH Calculations

This question tests your ability to calculate pH for solutions made by dissolving bases or mixing acids and bases.

Key Terms and Formulas

• $pOH = -\log[OH^-]$

• $pH + pOH = 14$

• For strong bases: $[OH^-] = \frac{\text{moles}}{\text{volume}}$

Step-by-Step Guidance

1. For part (a): Calculate moles of lithium hydroxide from mass and molar mass.

2. Find $[OH^-]$ by dividing moles by volume (in liters).

3. Calculate $pOH$ and then $pH$ using $pH + pOH = 14$.

4. For part (b): Calculate moles of HCl and mix with moles of NaOH, then determine excess $[H^+]$ or $[OH^-]$.

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Q4. The following reaction occurs in a 1.0 L flask at constant temperature:

Background

Topic: Chemical Equilibrium

This question tests your ability to use equilibrium constants ($K_c$) and initial concentrations to determine equilibrium concentrations and the direction of reaction.

Key Terms and Formulas

• Equilibrium constant ($K_c$): $K_c = \frac{[\text{products}]}{[\text{reactants}]}$

• ICE table: Initial, Change, Equilibrium concentrations.

Step-by-Step Guidance

1. Write the balanced equation and set up the ICE table for all species.

2. Determine the initial concentrations given in the problem.

3. Set up the equilibrium expression using $K_c$ and solve for the change in concentration ($x$).

4. Use the value of $K_c$ to determine the direction of the reaction (compare $Q$ to $K_c$).

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Q5. The double identity of nitrogen: a possibility for the reversal of ammonia and nitric acid synthesis

Background

Topic: Chemical Equilibrium and Reaction Direction

This question tests your understanding of equilibrium constants, reaction direction, and how changes in conditions affect equilibrium.

Key Terms and Formulas

• Equilibrium constant ($K_c$)

• Reaction quotient ($Q$)

• Le Chatelier's Principle

Step-by-Step Guidance

1. Write the equilibrium expression for the reaction.

2. Calculate $K_c$ using the provided concentrations.

3. Compare $Q$ to $K_c$ to determine the direction of the reaction.

4. Predict how changes in concentration or pressure would affect the equilibrium position.

Try solving on your own before revealing the answer!