Chemistry Study Material

Chemistry
1. What is the Kw of pure water at 50.0 ºC, if the pH is 6.630?
Soln
For purified water, [H+]= [OH-]
[H+]= 10^-6.630=2.34 x 10^-7 M = [OH-]
Kw = [H+][OH-]= (2.34 x 10^-7)(2.34 x 10^-7)= 5.50 x 10^-14
Therefore, answer is (b)
2. Find the percentage ionization of 0.337 M HF solution. The Ka for HF is 3.5*10^-4
Soln
HF <=> H+ + F-
Ka = [H+] x [F-] / [HF]
if x moles of HF disassociate we have
Ka = x^2 / .337 = 3.5 x 10 ^ -4
x^2 = 3.5 x 10 ^ -4 x .377 = 0.00013195
x = 0.0115 = [F-]
% ionization = .0115 / .337 % = 3.2%
answer E
3. Determine the Ka of an acid whose 0.294 M solution has a pH of 2.80
Soln
By presumptuous that the acid may be a monoprotic acid;
HA(aq) ⇌ H^+(aq) + A^-(aq)
we get the corresponding Ka expression as;
Ka = [H^+] [A^-] / [HA] (remember PORK - product Over Reactants = K)
we then calculate concentration from the pH:
pH = -log[H^+]
[H^+] = 10^-pH
[H^+] = 10^-2.80 = 1.58x10^-3 M = [A^-] (1:1 moles ratio)

We work the expression:
Ka = (1.58x10^-3) ² / 0.294 = 8.49x10^-6 ≈ 8.5x10^-6
Therefore, answer is D
4. Place the following in order of increasing acid strength
HBrO2, HBrO3, HBrO, HBrO4
Soln
There is a lot of O's within the BrOx particle, thus the more, the better it's to get rid of the
H as a result of the extremely negative O's pull powerfully on the shared electrons
Therefore, the correct order is;
HBrO < HBrO2 < HBrO3 < HBrO4
Answer is B
5. Which of the following series is amphoteric?
a. SO4 ion
b. HF
c. NH4 ion
d. HPO4 ion
e. KF
Soln
Amphoteric means that having the power to react with acids and bases. this means that
the substance can act as a base (acceptor of H+) towards associate degree acidic material,
associate degreed as an acid (donor of H+) towards a basic material. And of the higher
than answers solely ammonia exhibits each property. Others are simply neutral series
NH3 + H+ <==> NH4+ (reaction where NH3 acts as a base)
NH3 + NaH <===> NaNH2 + H2 (reaction where NH3 acts as an acid).
Therefore, NH3 is amphoteric.
Answer is C
6. Which one of the following acids will have the strongest conjugate base?
a. HCl
b. HClO2
c. H2SO4
d. HCN
e. KF
Soln

According to Lowry-Bronsted concept, a strong acid has weak conjugate base and a
weak acid has a strong conjugate base. Now, let us consider the stabilities of the
conjugate bases. Therefore, the acid with the strongest conjugate base, that is the most
stable anion is the strongest conjugate base.
The answer is HCN (D)
7. Determine ammonia concentration of an aqueous solution of pH of 11.0. The
equation for the dissociation of ammonia, NH3 (Kb=1.8*10^-5) is
NH3(aq) + H2O(l) =NH4 ion + OH ion
Soln
The ratio that exists between the equilibrium concentrations of the ammonium cations
and of the hydroxide anions and the equilibrium concentration of ammonia is given by
the base dissociation constant, Kb.
Kb=[NH+4] * [OH−] / [NH3]
at equilibrium, the solution will contain
[NH+4] = [OH−] = x M
The solution will also contain
[NH3] = (11−x) M
When x M ionizes, the initial concentration of ammonia will decrease by x M.
This means that the expression of the base dissociation constant will now take the form
Kb=x ⋅ x / 11.0−x
which is equal to
1.80*10^−5 = x * 11−x
Therefore,
11x -X^2 = 0.000018; -x^2 + 11x – 0.000018
X = (-11+/- 11.00) / -2
= 1.5 M
8. Which of the following solutions is a good buffer solution?
Soln
A buffer is composed of a mixture of a weak acid its conjugate base.