Sodium hydrogen carbonate (NaHCO₃) and sodium carbonate (Na₂CO₃) are used to prepare…
CHEMISTRY · Multi-part · AP · Chemistry · Acids and Bases · Acids and Bases
Problem
Sodium hydrogen carbonate (NaHCO₃) and sodium carbonate (Na₂CO₃) are used to prepare a buffer that is effective near pH 10.3. The two relevant acid–base equilibria for carbonic acid at 25 °C are described by pKa1=6.37 and pKa2=10.25.
In an experiment, 0.150 mol of Na₂CO₃ and 0.100 mol of NaHCO₃ are dissolved in water to make 1.00 L of buffer solution at 25 °C.
(a) Calculate the initial pH of the buffer.
(b) A 25.0 mL portion of 1.00 M HCl is added to the buffer with negligible volume change. Assuming complete reaction with the appropriate buffer component, calculate the new pH of the solution.
(c) Determine the percentage change in the hydronium-ion concentration as a result of the acid addition.
(d) Explain, with reference to the buffer components and their stoichiometric reaction with the added acid, why the pH change is relatively small compared with adding the same amount of HCl to pure water.