Strong base is dissolved in 565 mL of 0.400 M weak acid (Ka 4.20 x105) to make a
ID: 696218 • Letter: S
Question
Strong base is dissolved in 565 mL of 0.400 M weak acid (Ka 4.20 x105) to make a buffer with a pH of 4.19 Assume that the volume remains constant when the base is added. HA(aq)-OH-(aq) H2O(l)+ A-(aq) Calculate the pKa value of the acid and determine the number of moles of acid initially present. Number Number pk. 114.38 0.226 mol HA When the reaction is complete, what is the concentration ratio of conjugate base to acid? Number = 110.646 HA How many moles of strong base were initially added? Incorrect. Look in the bottom panel for more specific feedback Number 0.105 mol OHhow to do this part of the calculationExplanation / Answer
if ratio is
A-/HA = 0.646
and
HA initially = MV = 0.4*0.565 = 0.226
then
A- = 0.646HA
and we know
A- + HA = 0.226
substitute
0.646HA + HA = 0.226
HA = 0.226/(1+0.646)
HA = 0.13730
therefore,
A- = 0.646HA =0.646* 0.13730 = 0.0886958 mol of OH- was added