Chapter 6 - Acids + Bases

Question 1

Define a Bronsted-Lowry acid and base

Answer 1

Acids are proton donors
Bases are proton acceptors (have lone pair of electrons)

Water is always available to accept protons and donate protons

Question 2

What is a conjugate pair

Answer 2

An acid/base pair that differs by one proton
Strong acids have weak base pairs
Weak acids have strong base pairs

Question 3

Difference between dissociation constant K and pK

Answer 3

K and pK are inversely proportional
Large K means strong, large pKa means weak
Small K means weak, small pKa means strong
A small dissociation constant means weak acid or base
Kw = water constant
Ka = acid, ↑ Ka = strong acid, ↑ pKa = weak acid
Kb = base, ↑ Kb = strong base, ↑ pKb = weak base

Question 4

What does a buffer do in relation to pH

Answer 4

Buffers resist change in pH

A buffer is a weak acid and its salt or a weak base and its salt

Question 5

What is a good indicator for an acid-base titration

Answer 5

A weak acid or base with a pK close to pH of the equivalence point. KMnO4 is good because its purple but its reduced form is clear so colour fades immediately until no reducing agent is left in the mixture.

The indicator range should include the equivalence point rather than the end point. Indicator range should be where slope is steepest.

Question 6

Equations for pH and pOH

Answer 6

pH = pKa (at the equivalence point)
pH = pKa + log ([A-]/[AH]
pH = -log [H+]
[H+] = 10^-pH

pOH = pKb (at the equivalence point)
pOH = pKb + log ([OH-]/B]
pOH = -log[OH-]
[OH-] = 10^-pOH

Question 7

If a pKa is 5, at what pH will its concentration be 10^3 of its conjugate base?

Answer 7

pH = pKa + log ([A-]/[AH]) when ([A-]/[AH]) = 10^3
pH = pKa - 3
= 5 - 3 = 2

Question 8

An acid with pKa 5 has pKb of what in water?

Answer 8

In water pKa + pKb = 14

therefore 14 - 5 = 9

Question 9

State Le Chataliers Principle and result in pH

Answer 9

If protons are removed the reaction continues forward, increasing pH (making more basic)

Question 10

Why does pH drop drastically near equivalence point?

Answer 10

Because concentration of H+ increases sharply, pH drops = more acidic = increase in H+

Question 11

Why does HCl have a higher boiling point than H2 or Cl2?

Answer 11

a. HCl has greater mass
b. HCl is less polar
c. HCl is a smaller molecule
d. HCl has a weak dipole-dipole

d.

Question 12

Partial pressure of Cl2 is 35atm. Mole fraction of HCl is 0.4. The total pressure is 100atm. What is the partial pressure of H2?

Answer 12

mole fraction of 0.4 = 40atm
total pressure = 100
therefore 100 - (40+35) = 25atm

Question 13

Which has the highest pH?
H2SO4
NaHSO4
Na2SO4

Answer 13

highest pH = most basic

therefore Na2SO4 is most basic as no H+

Question 14

What is the pH of 0.001M HCl

Answer 14

pH = -log [0.001]
pH = -log 10^-3
pH = 3

Question 15

What results from combining a strong acid and strong base or a weak acid and weak base?

Answer 15

Complete dissociation = neutral pH