pH and Indicators
[ ] = concentration
- pH = -log10 [H+]
- pOH = -log10 [OH-]
Self Ionisation of Water
H2O ⇌ H+ + OH-
Ionic product of water (Kw) = [H+][OH-]
ie. [H+][OH-] = 1 x 10-14
Since [H+] = [OH-] then [H+]2 = 1 x 10-7
Ionic product of water (Kw) = [H+][OH-]
- At 25°C Kw is 1 x 10-14
ie. [H+][OH-] = 1 x 10-14
Since [H+] = [OH-] then [H+]2 = 1 x 10-7
- pH of pure water at 25°C = -log10[H+]
- pOH of pure water is also 7
- Solution with pH lower than 7 has greater [H+] than [OH-] i.e. acidic
- However these pH and pOH values apply only at 25°C. According to Le Chateliers principle, if the temperature is changed Kw value will change so pH of pure water will no longer be 7
- In pure water [H+] will always equal [OH-] so pure water is always neutral regardless of temperature
The pH Scale
Ranges 0-14 (values outside this are possible but not always accurate because even strong acid/bases don't dissociate fully in highly concentrated solutions)
pH values can be measured with universal indicator or pH meter
pH values can be measured with universal indicator or pH meter
pH of Strong Acids and Bases
eg. 1 calculate pH of i) 0.1M solution HCl
ii) 0.1M solution H2SO4
iii) solution with 2g NaOH in 500 cm3
ii) 0.1M solution H2SO4
iii) solution with 2g NaOH in 500 cm3
i) HCL -> H+ + Cl-
0.1 mole HCl -> 0.1 mole H+
pH = -log10[H+] = -log10(0.1) = 1
ii) H2SO4 -> 2H+ + SO42-
0.1 mole H2SO4 -> 0.2 mole H+
pH = -log10(0.2) = 0.7
iii) NaOh -> Na+ + OH-
2g/ 500CM3 -> 4g/l
no. mol = mass in g = 4 = 0.1
Mr 40
0.1 mole NaOH -> 0.1 mole OH-
pOH = -log10[OH-] = -log10(0.1) = 1
pH + pOH = 14
pH = 13
0.1 mole HCl -> 0.1 mole H+
pH = -log10[H+] = -log10(0.1) = 1
ii) H2SO4 -> 2H+ + SO42-
0.1 mole H2SO4 -> 0.2 mole H+
pH = -log10(0.2) = 0.7
iii) NaOh -> Na+ + OH-
2g/ 500CM3 -> 4g/l
no. mol = mass in g = 4 = 0.1
Mr 40
0.1 mole NaOH -> 0.1 mole OH-
pOH = -log10[OH-] = -log10(0.1) = 1
pH + pOH = 14
pH = 13
pH of Weak Acids/ Bases
Acid/ Base dissociation constants
HA ⇌ H+ + A-
Ka = [H+][A-] [HA] |
¦
¦ ¦ ¦ ¦ ¦ |
BOH
⇌ B+ + OH-
Kb = [B+][OH-] [BOH] |
Also [H+] =
√Ka x Macid
= √Kb x Mbase
e.g. 2 i) Write Ka expression of dissociation of ethanoic acid
ii) Calculate pH 0.2M solution ethanoic acid given its Ka value is 1.7 x 10-5
ii) Calculate pH 0.2M solution ethanoic acid given its Ka value is 1.7 x 10-5
i) CH3COOH
⇌ CH3COO- + OH-
Ka = [CH3COO-][OH-]
[CH3COOH]
ii) [H+] = √Ka x Macid
Ka = [CH3COO-][OH-]
[CH3COOH]
ii) [H+] = √Ka x Macid
√(1.7 x 10-5) x 0.2
= 1.84 x 10-3
pH = -log10 [H+]
-log10(1.84 x 10-3)
= 2.73
pH = -log10 [H+]
-log10(1.84 x 10-3)
= 2.73
e.g. i) Write Kb expression for amonium hydrixide dissociation in H2O
ii) A 0.015M solution NH4OH has pH 10.7. Calculate Kb
ii) A 0.015M solution NH4OH has pH 10.7. Calculate Kb
i) NH4OH
⇌ NH4+ + OH-
Kb = [OH-][NH4+]
[NH4OH]
ii) pOH = 14 - 10.7 = 3.3
-log10[OH-] = 3.3
log10[OH-] = -3.3
[OH-] = inverse log (-3.3) = 5 x 10-4
[OH-] = √Kb x Mbase
Kb = [OH-][NH4+]
[NH4OH]
ii) pOH = 14 - 10.7 = 3.3
-log10[OH-] = 3.3
log10[OH-] = -3.3
[OH-] = inverse log (-3.3) = 5 x 10-4
[OH-] = √Kb x Mbase
(5 x 10-4)2 = (√Kb(0.015))2
0.00000025 = Kb(0.015)
0.00000025 = Kb
0.015
1.68 x 10-5 = Kb
0.00000025 = Kb
0.015
1.68 x 10-5 = Kb
Acid/Base Indicators
Most indicators are weak acids and dissociate as follows
HIn ⇌ H+ + In-
(red) (blue)
HIn ⇌ H+ + In-
(red) (blue)
- Solution acidic. Equilibrium of indicator lies on left to remove H+
- Solution alkaline. Equilibrium of indicator lies on right to increase H+
- Range of indicator is the pH range over which there is a clear colour change
Indicator:
Methyl Orange Phenophtalen Litmus |
pH Range:
3-5 8-10 5-8 |
Colours:
Red (below 3) Colourless (below 8) Red (below 5) |
Yellow (above 5) Pink (above 10) Blue (above 8) |
Titration Type:
Stong Acid/ Strong Base Strong Base/ Weak Base Weak Acid/ Strong Base |
pH Change at End:
3-10 3-7 7-10 |
Suitable:
All three Methyl Orange Phenolphtalein |
Unsuitable:
None Phenolphtalein (end point reached before pink) Methyl Orange (end point reached before yellow) |
Limitations of the pH scale:
- It does not work with concentrated solutions as once conc. goes above 1M complete dissociation does not always occur and therefore any calculations are inaccurate
- pH is limited to the 0-14 scale and solutions in water