Give the pH for the following solutions below: a) A buffer is prepared by making
ID: 911197 • Letter: G
Question
Give the pH for the following solutions below:
a) A buffer is prepared by making 750.00 mL of a solution that is both .300 M solution of hypochloric acid (HOCl) and 0.250 M sodium hypochlorite. What is the pH of this solution after 5.0 mL of 5.0 M HCl has been added given Ka for hypochloric acid = 3.0x10-8
b) The titration mixture starting with 250.00 mL of 0.5 M nitric acid after 50.0 mL of 0.3 M sodium hydroxide has been added.
c) 0.20 M potassium cyanide. (Ka for hydrocyanic acid is 6.71x10-10)
Explanation / Answer
a)
millimoles of acid = 750 x 0.3 = 225
millimoles of salt = 750 x 0.250 = 187.5
millimoles of additional added strong acid = C = 5 x 5 = 25
pKa = -logKa = -log (3.0x10-8) = 7.52
on addition of ’ C’ moles of acid to acidic buffer salt moles decreases and acid moles increases
so
pH = pKa + log [Salt –C/acid + C]
pH = 7.52 + log [225 –25 / 187.5 + 25 ]
pH = 7.49
b)
millimoles of HNO3 = 250 x 0.5 = 125
millimoles of NaOH = 50 x 0.3 = 25
acid millimoles > base millimoles
[H+] = 125 - 25 / (250+50) = 0.333M
pH = -log [H+]
pH = 0.477
c)
C = 0.20 M
KCN is the salt of weak acid + strong base
pKa = 9.17
pH = 7 + 1/2 [pKa + logC]
pH = 7 + 1/2 [9.17 + log 0.20]
pH = 11.23
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