Table of contents

1. Intro to General Chemistry3h 48m
2. Atoms & Elements4h 16m
3. Chemical Reactions4h 10m
4. BONUS: Lab Techniques and Procedures1h 38m
5. BONUS: Mathematical Operations and Functions48m
6. Chemical Quantities & Aqueous Reactions3h 53m
7. Gases3h 49m
8. Thermochemistry2h 37m
9. Quantum Mechanics2h 58m
10. Periodic Properties of the Elements3h 9m
11. Bonding & Molecular Structure3h 29m
12. Molecular Shapes & Valence Bond Theory1h 57m
13. Liquids, Solids & Intermolecular Forces2h 23m
14. Solutions3h 1m
15. Chemical Kinetics2h 53m
16. Chemical Equilibrium2h 29m
17. Acid and Base Equilibrium5h 1m
18. Aqueous Equilibrium4h 47m
19. Chemical Thermodynamics1h 50m
20. Electrochemistry2h 42m
21. Nuclear Chemistry2h 36m
22. Organic Chemistry5h 7m
23. Chemistry of the Nonmetals2h 39m
24. Transition Metals and Coordination Compounds3h 16m

18. Aqueous Equilibrium

Titrations: Weak Acid-Strong Base

General Chemistry

18. Aqueous Equilibrium

Titrations: Weak Acid-Strong Base

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Multiple Choice

A 10.0 mL sample of 0.200 M hydrocyanic acid (HCN) is titrated with 0.0998 M NaOH. What is the pH at the equivalence point? For hydrocyanic acid, pKₐ = 9.31.

11.00

8.50

11.50

7.00

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Verified step by step guidance

Start by understanding that at the equivalence point in a titration, the amount of acid equals the amount of base added. This means that all the hydrocyanic acid (HCN) will be converted to its conjugate base, cyanide ion (CN⁻).

Calculate the initial moles of HCN using the formula: \( \text{moles} = \text{concentration} \times \text{volume} \). For HCN, use \( 0.200 \text{ M} \times 10.0 \text{ mL} \). Remember to convert mL to L by dividing by 1000.

Determine the volume of NaOH needed to reach the equivalence point using the stoichiometry of the reaction. Since the reaction is 1:1, the moles of NaOH required will be equal to the moles of HCN calculated in the previous step. Use the formula: \( \text{volume} = \frac{\text{moles}}{\text{concentration}} \) for NaOH.

At the equivalence point, the solution contains the conjugate base CN⁻. Use the formula for the hydrolysis of the conjugate base to find the pH: \( \text{pH} = 14 - \frac{1}{2}(\text{pK}_a + \text{pK}_w - \text{pK}_b) \). Calculate \( \text{pK}_b \) using \( \text{pK}_b = 14 - \text{pK}_a \).

Finally, substitute the values into the hydrolysis equation to find the pH at the equivalence point. This involves calculating the concentration of CN⁻ and using the \( \text{pK}_b \) value derived from \( \text{pK}_a \).