Question

CHEM 104 Homework 7 M. Integrative Problems 1. (15 points): An environmental engineer makes a buffer by combining 85.5 8 of Na кнсо, in 2.00 L of water. Te pk, of HC03-is 10.33. a) What is the pH of this buffer? b) The engineer wants to adjust the buffer pH to 10.00 by adding a CO, and 68.4 8 either concentrated HCI or g) is (approximately 12.1 M) or solid NaOH. Which should they add, needed to adjust the pH to 10.00? and how much (mL

Answer 1

Mass of Na_2 co_3 = 85.5g molar mass of Na_2 CO_3 = 105.99 g/mol moles of Na_2 CO_3 = mass/molar mass = 85.5g/105.99 g/mol = 0.8067 mol volume of solution = 2.00L concentration of Na_2 CO_3 = moles/volume (L) = 0.2067 mol/2.00L = 0.21034M Na_2 CO_3 dissociates completely in solution Na_2 CO_3 (aq) rightarrow 2Na^+ (aq) + CO^2 -_3 (aq) mass of KHCO_3 = 68: 4g molar mass of KHCO_3 = 100.2 g/mol moles of KHCO_3 = mass/molar mass = 68.4g/100.12 g/mol = 0.6832mol volume of solution = 2.00L concentration of KHCO_3 = moles/volume (L) = 0.6832 mol/2.00L = 0.3416M KHCO_3 dissociates completely in solution KHCO_3 (aq) rightarrow K^+ (aq) + HCO^-_3 (aq) implies [HCo^-_3] = 0.3416M pka of HCO^-_3 = 10.33 Using Henderson Hasselbach equation pHc = pka + log [CO^2 -_3]/[HCO^-_3] pH = 10.33 + log(0.4034M/0.3416M) pH = 10.40 Hence, pH of the buffer is 10.40 \r\n b) As the pH is decreasing