This question is about pH - AQA - A-Level Chemistry - Question 6 - 2021 - Paper 1
Question 6
This question is about pH.
Pure water dissociates slightly.
H₂O(l) ⇌ H⁺(aq) + OH⁻(aq)
ΔH = +57 kJ mol⁻¹
The equilibrium constant, Kc = \frac{[H^+][OH^-]}{[H_2O]}
... show full transcript
Worked Solution & Example Answer:This question is about pH - AQA - A-Level Chemistry - Question 6 - 2021 - Paper 1
Step 1
[H₂O] is not shown in the Kc expression.
96%
114 rated
Only available for registered users.
Sign up now to view full answer, or log in if you already have an account!
Answer
[H₂O] is not included in the Kc expression because its concentration is virtually constant in dilute solutions, thus simplifying the calculations without affecting the equilibrium.
Step 2
Explain why the value of Kw increases as the temperature increases.
99%
104 rated
Only available for registered users.
Sign up now to view full answer, or log in if you already have an account!
Answer
The value of Kw increases with temperature because the dissociation of water is an endothermic process. As temperature rises, the equilibrium shifts to the right, producing more H⁺ and OH⁻ ions.
Step 3
Give the expression for pH.
96%
101 rated
Only available for registered users.
Sign up now to view full answer, or log in if you already have an account!
Answer
The expression for pH is given by:
pH = -\log[H^+]
Step 4
Calculate the pH of pure water at 50 °C.
98%
120 rated
Only available for registered users.
Sign up now to view full answer, or log in if you already have an account!
Answer
At 50 °C, using the value of Kw from Table 5, we have:
Kw = [H⁺][OH⁻] = 5.48 × 10⁻¹⁴mol² dm⁻⁶.
Since [H⁺] = [OH⁻] in pure water, we can set [H⁺] = [OH⁻] = x:
Sign up now to view full answer, or log in if you already have an account!
Answer
Water is considered neutral at 50 °C because at this temperature, [H⁺] and [OH⁻] are equal, meaning the solution maintains a pH of approximately 6.13, reflecting neither acidic nor basic conditions.
Step 6
Use Figure 3 to give the true pH value when the pH meter reading is 5.6.
97%
121 rated
Only available for registered users.
Sign up now to view full answer, or log in if you already have an account!
Answer
From Figure 3, a pH meter reading of 5.6 corresponds to a true pH value of approximately 5.5.
Step 7
Suggest why the pH probe is washed with distilled water between each of the calibration measurements.
96%
114 rated
Only available for registered users.
Sign up now to view full answer, or log in if you already have an account!
Answer
The pH probe is washed with distilled water to avoid contamination from previous solutions, which could affect the accuracy of the pH readings.
Step 8
Explain why the volume of sodium hydroxide solution added between each pH measurement is smaller as the end point of the titration is approached.
99%
104 rated
Only available for registered users.
Sign up now to view full answer, or log in if you already have an account!
Answer
As the endpoint of the titration is approached, the solution's pH changes more rapidly with the addition of NaOH. Therefore, smaller volumes must be added to avoid overshooting the endpoint.
Step 9
State why all three of the indicators in Table 6 are suitable for this titration.
96%
101 rated
Only available for registered users.
Sign up now to view full answer, or log in if you already have an account!
Answer
All three indicators change colour within the pH range of the titration, providing clear visual cues at the endpoint, which is crucial for accurate determination.
Step 10
Calculate the pH of the final solution at 25°C.
98%
120 rated
Only available for registered users.
Sign up now to view full answer, or log in if you already have an account!
Answer
Total moles of HCl initially = concentration × volume = 0.150 mol/dm³ × 0.025 dm³ = 0.00375 mol.
Total moles of NaOH added = 0.200 mol/dm³ × (36.25/1000 dm³) = 0.00725 mol.
Moles of NaOH remaining = 0.00725 - 0.00375 = 0.0035 mol.
Final total volume = 25.00 cm³ + 36.25 cm³ = 61.25 cm³ = 0.06125 dm³.
