Acids & Bases

Cheat Sheet  ·  Chemistry Gr 11
Emil Oliversen
pH & pOH
pH
−log[H⁺]
[H⁺]
10^(−pH)
pOH
−log[OH⁻]
[OH⁻]
10^(−pOH)
Sum
pH + pOH = 14 (25°C)
Kw
[H⁺][OH⁻] = 1.0×10⁻¹⁴
pH SCALE
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
← acidic [H⁺]>10⁻⁷ neutral=7 basic [OH⁻]>10⁻⁷ →
Substance≈ pH
Gastric acid1
Lemon juice2
Coffee5
Pure water7
Blood7.4
Baking soda8.3
Bleach12
STRONG ACIDS & BASES (fully dissociate)
Strong acids (memorise):
HCl, HNO₃, H₂SO₄, HBr, HI, HClO₄
[H⁺] strong
C × n_H   (n_H = H⁺/molecule)
mono (HCl)
n_H = 1 → [H⁺] = C
diprotic (H₂SO₄)
n_H = 2 → [H⁺] = 2C
triprotic (H₃PO₄)
n_H = 3 → [H⁺] = 3C
Strong bases:
NaOH, KOH, LiOH, Ba(OH)₂, Ca(OH)₂
[OH⁻] = C × n_OH
WEAK ACIDS/BASES (partial dissociation)
  • CH₃COOH, H₂CO₃, HF, H₂S, NH₃
  • Use Ka or Kb + ICE table for [H⁺]
  • Ka = [H⁺][A⁻] / [HA]
  • Larger Ka = stronger weak acid
ICE TABLE
HAH⁺A⁻
InitialC00
Change−x+x+x
Equil.C−xxx
Ka
x² / (C−x) ≈ x²/C if x<5%
approx. x
√(Ka × C)
NEUTRALIZATION & TITRATION
Neutralization: acid + base → salt + water
Net ionic: H⁺(aq) + OH⁻(aq) → H₂O(l)
TITRATION (1:1 monoprotic)
CₐVₐ = CᵦVᵦ
mol acid = mol base at equiv.
General
nₐCₐVₐ = nᵦCᵦVᵦ
nₐ, nᵦ = moles of H⁺/OH⁻ per formula unit
AT EQUIVALENCE POINT
CombinationpH
Strong acid + strong base= 7
Strong acid + weak base< 7
Weak acid + strong base> 7
COMMON INDICATORS
IndicatorpH rangeColour change
Phenolphthalein8.2–10colourless→pink
Methyl orange3.1–4.4red→yellow
Litmus5–8red→blue
Bromothymol blue6–7.6yellow→blue
COMMON ACIDS & THEIR H⁺
HCl
1 H⁺ (monoprotic)
HNO₃
1 H⁺ (monoprotic)
H₂SO₄
2 H⁺ (diprotic)
H₂CO₃
2 H⁺ (diprotic)
H₃PO₄
3 H⁺ (triprotic)
CH₃COOH
1 H⁺ (weak acid)
COMMON MISTAKES
  • Weak acid ≠ dilute acid — strength = dissociation, not conc.
  • H₂SO₄ diprotic: 0.1 M → [H⁺] = 0.2 M (×2!)
  • Increasing pH means DECREASING [H⁺]
  • pOH = 14 − pH (never 14 + pH)
  • At equivalence: mol H⁺ = mol OH⁻ — check stoichiometry for diprotic
  • Weak acid ICE: x ≈ √(Ka×C) only valid if x/C < 5%
WORKED EXAMPLES
0.050 M HCl:
[H⁺]=0.050 M → pH=−log(0.050)=1.30
0.010 M NaOH:
[OH⁻]=0.010 → pOH=2 → pH=12
CH₃COOH 0.10 M, Ka=1.8×10⁻⁵:
x=√(1.8×10⁻⁵×0.10)=1.34×10⁻³ M
pH=−log(1.34×10⁻³)=2.87
Titration: 25.0 mL HCl + 31.5 mL 0.100 M NaOH:
Cₐ=0.100×31.5/25.0=0.126 M