An environmental chemist is monitoring a freshwater lake whose pH is naturally…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
An environmental chemist is monitoring a freshwater lake whose pH is naturally regulated by the carbonate buffer system consisting primarily of H₂CO₃ and HCO₃⁻ at 25 °C. A 1.00 L water sample from the lake contains 2.5 mmol of H₂CO₃ and 2.5 mmol of HCO₃⁻.
The following sequence of events occurs during transport of the sample to the laboratory:
Step 1: The sample is diluted by 50 % with deionized water to improve instrument sensitivity.
Step 2: The sample warms from 25 °C to 35 °C in the transport vehicle. The higher temperature increases the acid dissociation constant, causing a slight decrease in pK_a for the H₂CO₃ ⇌ H⁺ + HCO₃⁻ equilibrium.
Step 3: In the laboratory, 1.0 mL of 0.010 M NaOH is accidentally introduced into the diluted, warmed sample. The volume change is negligible.
Using a systematic, step-by-step qualitative analysis, address the tasks below. Cite the key chemical species involved at each stage and explain how their amounts or ratios change.
(1) Describe qualitatively how the 50 % dilution in Step 1 affects (i) the absolute concentrations of buffer components and (ii) the pH of the solution. Justify your answer.
(2) Explain how the temperature increase in Step 2 influences the position of the carbonate equilibrium and discuss whether this change alone would be expected to raise, lower, or leave unchanged the sample’s pH. Provide reasoning in terms of Le Châtelier’s principle.
(3) After Steps 1 and 2, predict the effect of the inadvertent NaOH addition in Step 3 on the pH of the sample. State whether the pH change will be large or small relative to the change that would occur if the same NaOH were added to pure water initially at the lake’s pH. Justify fully.
(4) Synthesize your analyses from parts (a)–(c) to predict whether the final pH of the processed sample is expected to be higher than, lower than, or approximately equal to the original lake water pH. Support your conclusion with a coherent argument that references all three steps.