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

2. Atoms & Elements

Calculating Molar Mass

General Chemistry

2. Atoms & Elements

Calculating Molar Mass

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Multiple Choice

A 5.65 g sample of a weak acid with Ka=1.3×10⁻⁴ was combined with 5.20 mL of 5.90 M NaOH and the resulting solution was diluted to 750 mL. The measured pH of the solution was 4.20. What is the molar mass of the weak acid?

210.4 g/mol

120.1 g/mol

180.3 g/mol

150.2 g/mol

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Verified step by step guidance

First, identify the chemical reaction taking place. The weak acid reacts with NaOH to form its conjugate base and water. Write the balanced chemical equation for this reaction.

Calculate the moles of NaOH used in the reaction. Use the formula: \( \text{moles of NaOH} = \text{volume (L)} \times \text{molarity (M)} \). Convert the volume from mL to L before calculation.

Determine the moles of the weak acid that reacted with NaOH. Since NaOH is a strong base, it will react completely with the weak acid. The moles of NaOH will be equal to the moles of the weak acid that reacted.

Calculate the concentration of the conjugate base formed after the reaction. Use the formula: \( \text{concentration} = \frac{\text{moles of conjugate base}}{\text{total volume of solution (L)}} \). The total volume is 750 mL, converted to liters.

Use the Henderson-Hasselbalch equation to relate the pH, pKa, and the ratio of the concentrations of the conjugate base and the weak acid. Rearrange the equation to solve for the initial concentration of the weak acid, and then use this to find its molar mass by dividing the mass of the sample by the moles of the weak acid.