Brønsted-Lowry Acids and Bases
pH and pOH
Determining Ka and Kb
100
The term for a species that is capable of either donating or accepting protons
amphiprotic or amphoteric
100
What is the pH of a solution with a pOH of 7.2?
pH + pOH = 14
pH + 7.2 =14
14 - 7.2 = pH = 6.8
100
Use the Kb for the nitrite ion, NO2−, to calculate the Ka for its conjugate acid.
Kb= 2.17 × 10−11
𝐾a×𝐾b=1.0×10−14=𝐾w
𝐾a=𝐾w/𝐾b=1.0×10−14/2.17×10−11=4.6×10−4
200
What are the hydronium ion concentration and the hydroxide ion concentration in pure water at 25 °C?
𝐾w=[H3O+][OH−]=(𝑥)(𝑥)=𝑥2=1.0×10−14
so:
𝑥=[H3O+]=[OH−]=1.0×10−7
200
What is the equation used to relate pH and [H3O+]?
pH = -log[H3O+]
200
Determine the relative acid strengths of NH4+ and HCN by comparing their ionization constants. HCN ionization constant is 4.9 × 10−10. The ionization of NH4+ conjugate base, NH3, is 1.8 × 10−5.
NH4+ is the slightly stronger acid.
Ka for NH4+=5.6×10−10
300
A solution of an acid in water has a hydronium ion concentration of 2.0 × 10−6 M. What is the concentration of hydroxide ion at 25 °C?
[OH−]=𝐾w/[H3O+]=1.0×10−14/2.0×10−6=5.0×10−9
300
What is the pH of stomach acid, a solution of HCl with a hydronium ion concentration of 1.2 × 10−3 M?
pH=−log[H3O+]
=−log(1.2×10−3)
=−(−2.92)=2.92
300
At equilibrium, a solution contains [CH3CO2H] = 0.0787 M and [H3O+]=[CH3CO2−]=0.00118𝑀. What is the value of Ka for acetic acid?
CH3CO2H(𝑎𝑞)+H2O(𝑙)⇌H3O+(𝑎𝑞)+CH3CO2−(𝑎𝑞)
𝐾a=[H3O+][CH3CO2−]/[CH3CO2H]
=(0.00118)(0.00118)/(0.0787)=1.77×10−5
400
What are the two equations that represent HSO3- as an acid with OH- and a base with HI?
HSO3−(𝑎𝑞)+OH−(𝑎𝑞)⇌SO32−(𝑎𝑞)+H2O(𝑙)HSO3−
HSO3−(𝑎𝑞)+HI(𝑎𝑞)⇌H2SO3(𝑎𝑞)+I−(𝑎𝑞)
400
Calculate the hydronium ion concentration of blood, the pH of which is 7.3.
pH=−log[H3O+]=7.3
log[H3O+]=−7.3
[H3O+]=10−7.3
[H3O+]=5×10−8𝑀
400
What is the equilibrium constant for the ionization of the HPO42−ion, a weak base
HPO42−(𝑎𝑞)+H2O(𝑙)⇌H2PO4−(𝑎𝑞)+OH−(𝑎𝑞)
if the composition of an equilibrium mixture is as follows: [OH−] = 1.3 × 10−6 M; [H2PO4−]=0.042𝑀; and [HPO42−]=0.341𝑀?
Kb = [OH-][H2PO4−] / [HPO42−]
Kb for HPO42−=1.6×10−7
500
What is the conjugate base of NH4+?
NH3
500
What are the pOH and the pH of a 0.0125-M solution of potassium hydroxide, KOH?
pOH=−log[OH−]=−log0.0125
=−(−1.903)=1.903
pH+pOH=14.00
pH=14.00−pOH=14.00−1.903=12.10
500
Find the concentration of hydroxide ion, the pOH, and the pH of a 0.25-M solution of trimethylamine, a weak base:
(CH3)3N(𝑎𝑞)+H2O(𝑙)⇌(CH3)3NH+(𝑎𝑞)+OH−(𝑎𝑞)
𝐾b=6.3×10−5
𝐾b=(𝑥)(𝑥)(0.25−𝑥)=6.3×10−5, 𝑥=4.0×10−3𝑀=[OH−]
pOH=−log(4.0×10−3)=2.40
pH=14.00−pOH=14.00−2.40=11.60