A biochemical protocol calls for 1.00 L of a buffer solution with pH 9.20 and an…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A biochemical protocol calls for 1.00 L of a buffer solution with pH 9.20 and an overall concentration of 0.300 M based on the ammonia/ammonium ion system. The pKa of the ammonium ion (NH4+) is 9.25 at 25 °C.
Available reagents:
• Concentrated NH3(aq), 14.8 M
• Solid NH4Cl, molar mass 53.49 g·mol⁻¹
Because measuring very small volumes accurately is difficult, the chemist first prepares 250.0 mL of a 2.00 M NH3(aq) solution by diluting the concentrated stock, and then uses that intermediate solution to make the final buffer.
(1) Determine the required ratio [NH3]/[NH4+] in the final buffer that will produce the target pH.
(2) Calculate the number of moles of NH3 and NH4+ that must be present in the 1.00 L of buffer with the stated total concentration.
(3) Compute the mass of NH4Cl that must be dissolved to supply the needed amount of NH4+.
(4) Determine (i) the volume of the 2.00 M NH3 solution that must be transferred into the buffer flask, and (ii) the volume of 14.8 M NH3 stock solution required to prepare 250.0 mL of the 2.00 M intermediate solution.
(5) Qualitatively explain how incomplete dissolution of the NH4Cl before the NH3 solution is added would affect the pH of the final mixture and why.