Directions. You may use your textbook and own notes but no other source. You may

Question

Directions. You may use your textbook and own notes but no other source. You may ask questions only of the instructor, no one else. 1. A monoprotic weak acid, HA, 100g, 100. g/mol) is titrated with NaOH (0.250 M). After the addition 20.0 ml of NaOH the pH was 5.81. Calculate the initial moles of HA, the moles of NaOH added, the moles of HA and A- after the addition of the addition of the NaOH and the pKa of HA. Use the Henderson Hasselbach eq. HA(aq) + OH-(aq) A-(aq) + H2O mol HAinitial = _________ mol NaOH added = mol HA after added = _______ mol A- after NaOH added = _________ pKa = __________

Explanation / Answer

Initial moles of HA = 1/100 = 0.01 moles

Moles of NaOH added = 0.25*20/1000 = 5x10-3 moles

pH = pKa + log[A-]/[HA]

After adding 0.005 mooles of NaOH, there will be 0.005-x moles of HA and 0.005+x moles of A-

Since the pH of the solution is 5.81, the [H+] = 1.55x10-6 M, which is also equal to x

Therefore, moles of HA after NaOH added is 0.005

moles of A- after NaOH added is 0.005

pKa = pH -log (0.005/0.005) = 5.81.

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