In: Chemistry
1.Calculate the pH of a 0.075 M soultion of hydrazoic acid, which is a weak acid. Hydrazoic acid, HN3, has an acid dissociation constant Ka, equal to 1.9 x 10-5. Also, calculate the concentrations of the azide ion (N3-) and the hydroxide ion.
2. Calculate the pH that would result if 15.0 mL of 0.075 M NaOH, strong base, were added to 15.0mL of the solution in the problem above.
I have already completed 1, just need help with 2. If you can also show your work I would greatly appreciate it. Please and thank you.
1)For a weak HN3 acid we can write,
HN3 < ----------- > H+ + N3- .
Initial concentrations 0.075M 0 0
Conc. At eqm. (0.075 – X) X X
Dissociation constant Ka is given as,
Ka =[H+][N3-]/[HN3]………..(1)
Ka = 1.9 x 10-5 and [H+] = X moles/L, [N3-]= X moles/L, [HN3] = (0.075 – X).
Hence, eq.1 takes form
1.9 x 10-5 = X x X / (0.075-X)……………..(2)
As, HN3(hydrazoic acid) is weak acid it ionizes little hence, X <<< 0.075 so (0.075 – X) = 0.0759(apprx.)
Eq.2 takes form,
1.9 x 10-5 = X2/(0.075)
X2 =1.9 x 10-5 x 0.075
X2 = 1.425 x 10-6
X = 1.19 x 10-3 moles/L
i.e. [H+] = X = 1.19 x 10-3 moles/L and also [N3-] = X = 1.19 x 10-3 moles/L
pH = -log[H+]
pH = -log(1.19 x 10-3)
pH = 2.92
Hence pH of 0.075 M HN3 (hydrazoic acid) is 2.92.
We know that
pH + pOH = 14
pOH = 14 – pH
pOH = 14 – 2.92
pOH = 11.08
-log[OH-] = 11.08
log[OH-] = -11.08
Taking antilog of both sides,
[OH-] = Antilog(-11.08)
[OH-] = 8.32 x 10-12 moles/L.
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2)
NaOH + HN3 < -------- > NaN3 + H2O.
a)Determine milimoles of acid and bases before reaction,
NaOH = Molarity x volume = (0.075M) x (15mL) = 1.125 milimoles
HN3 = molarity x volume = (0.075M) x (15mL) = 1.125 milimoles.
Addition of equal Milimoles of weak acid and strong base will give the solution whose solution is numerically equal to pKa of acid
Hence,
pH = pKa of HN3 acid
pH = pKa = -log(Ka)
pH = -log(Ka)
pH = -log(1.9 x 10-5)
pH = 4.72
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