Calculated pH: X-I-X quadratic Measured pH: PH paper 10.5, 2.99, 3.31, 3.82 Weak Acid-Base Molarity: 0.0010 M NH4OH 0.0010 M CH3COOH 0.010 M CH3COOH 0.01 M CH3COOH 0.0010 M CH3COOH 0.0010 M CH3COOH 0.00010 M CH3COOH 0.0001 M CH3COOH 0.0001 M CH3COOH Salts Molarity: 0.1 M NaCl 0.1 M NaCH3COO 0.1 M NH4CH3COO 0.1 M NH4CH3COO 0.1 M NaHCO3 0.1 M Na2CO3 Calculated pH: 0.10 M NaCl – 8.81 0.10 M NaCH3COO – 7.02 0.10 M NH4CH3COO – 9.62 0.10 M NaHCO3 – 4.80E-11 0.10 M Na2CO3 – 2.40E-08 0.10 M Na2CO3 – 2.10E-04 Buffer Molarity: CH3COOH/NaCH3COO – 1.80E-05 Calculated pH: 4.69 Measured pH: 4.12 pH after 30 drops of HCl: 5.31 pH after 30 drops of NaOH: 4.12
The Correct Answer and Explanation is:
Correct Answer and Key Observations:
- Weak Acid-Base Solutions:
- For weak acid solutions such as acetic acid (CH₃COOH), pH increases as the molarity decreases. This is consistent with dilution reducing the concentration of hydrogen ions.
- Example: 0.010 M CH₃COOH has a higher hydrogen ion concentration than 0.0001 M CH₃COOH, thus its pH will be lower.
- NH₄OH, a weak base, at 0.0010 M should yield a pH slightly above 7. The value 10.5 is consistent with this weak basic behavior.
- Salt Solutions:
- NaCl is a neutral salt from a strong acid and base. The calculated pH of 8.81 seems slightly high, possibly due to water purity or instrumentation error, as NaCl alone should yield pH ≈ 7.
- NaCH₃COO results from a strong base and weak acid, producing a basic solution. A pH of 7.02 is consistent with this.
- NH₄CH₃COO is a salt of a weak acid and weak base. The calculated pH of 9.62 indicates the basic component dominates.
- NaHCO₃ and Na₂CO₃ are basic due to the carbonate and bicarbonate ions. The extremely low calculated values (e.g., 4.80E-11) likely represent [H⁺], not pH. Converting gives pH ≈ 10.3 and 7.6, respectively.
- Buffer System:
- A buffer made from CH₃COOH and NaCH₃COO should resist changes in pH when small amounts of acid or base are added.
- The measured pH of 4.12 vs. calculated 4.69 shows that the actual buffer was slightly more acidic.
- After 30 drops of HCl, the buffer’s pH increased to 5.31, possibly due to a mislabeling or unexpected chemical behavior. Normally, acid addition should decrease pH.
- After 30 drops of NaOH, the pH returned to 4.12, which also indicates possible experimental inconsistency.
Conclusion:
This data aligns generally with acid-base and buffer theory. Discrepancies between calculated and measured values can stem from experimental error, concentration inaccuracies, or instrumentation limitations like pH paper precision.
