Lecture 19 - pH and buffers

Question 1

Concentration (2)

Answer 1

• Weight / volume = g/L = weight per unit volume = molecular concentration
e.g., 9 g L-1 NaCl
Could also be expressed as 0.9% NaCl (0.9 g per 100 ml)
• Volume / volume
e.g., 70% ethanol (70 ml per 100 ml solution)

Question 2

Mole (2)

Answer 2

Mole = The atomic or molecular weight of a substance multiplied by one gram.
Number of moles = mass in grams / MW

Question 3

Molarity (3)

Answer 3

Molarity = The number of moles per litre of solution.
Molarity = number of moles [mass / MW] / volume
A solution containing 1 mol L-1 of a substance would be described as 1 molar (abbreviated M).

Question 4

Acid ()

Answer 4

Acid: a substance that produces H+ when dissolved in water (e.g., HCl, H2SO4)
HCl 		 	H+ + Cl-   	(a strong acid)
CH3COOH 	 	H+ + CH3COO-  (a weak acid)
More generally,
HA 		 	H+ + A-

Question 5

Acid dissociation constant (1)

Answer 5

Ka = [H+] [A-} / [HA]

Question 6

pH = Hydrogen ion concentration (1)

Answer 6

pH = log (1/[H+]) = -log([H+])

Question 7

Basic pH examples (2)

Answer 7

Household bleach

Household ammonia

Question 8

Neutral pH examples (6)

Answer 8

Milk
Saliva
Human blood/tears
Sea water
Egg white
Solution of baking soda

Question 9

Acidic pH examples (8)

Answer 9

Black coffee
Beer
Tomato juice
Red wine
Cola
Vinegar
Lemon juice
Gastric juice

Question 10

pH of water (1)

Answer 10

Ka = [H+] [OH-] / [H2O] = 1.8X10-16 M

Question 11

Kw (working out and formula) (2)

Answer 11

Kw = [H+] [OH-] = 1.0 x 10-14
[H2O} = mass/mr = 100g / 18 = 55.56
Mass = 1 L = 1000g
= [H+] [OH-] / 55.56 = 1.8X10-16 M
Rearrange to get KW = 55.56 x 1.8 x 10-16
= 1.0 x 10-14

Question 12

pKa (1)

Answer 12

pKa = -log(Ka)

Question 13

Henderson-Hasselbalch Equation (5)

Answer 13

Check formula sheet