6 12/17/2016 10:55 PM 6/10 12/8/2016 11:49 PM Print Calculator Periodic Table Qu

Question

6 12/17/2016 10:55 PM 6/10 12/8/2016 11:49 PM Print Calculator Periodic Table Question 6 of 13 Assume that the volume remains constant when the base is added. Calculate the pKavalue of the acid and determine the number of moles of acid initially present. Numbe Number p K mol HA When the reaction is complete, what is the concentration ratio of conjugate base to acid? Number HA How many moles of strong base were initially added? Number mol OH Previous Give Up & View Solution Check Answer Next I Ex Gradebook 09% A

Explanation / Answer

pKa = -log Ka
= -log (3.09*10^-5)
= 4.51

mole of weak acid present initially,
mol = M*V
= 0.400 M * 0.745 L
= 0.298 mol

use:
pH = pKa + log {[A-]/[HA]}
4.16 = 4.51 + log ([A-]/[HA])
log ([A-]/[HA])= -0.35
[A-]/[HA] = 0.447

let the number of base be x mol
then,
[A-]/[HA] = x/ (0.298-x) = 0.447
x = 0.133 - 0.447*x
x = 0.0919 mol
Answer: 0.0919 mol OH-

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