كيف يحسب الرقم الهيدروجيني pH للمحاليل؟ How to calculate pH and pOH?

يحسب الرقم الهيدروجيني بالعلاقة: pH = -log[H⁺]، وللمحاليل المائية عند 25°C: pH + pOH = 14. إذا كان pH 7 فالمحلول قاعدي. [الصيغة الرمزية الموزونة: pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴]

R3.1Reactivity 3 — What are the mechanisms of chemical change?

Proton transfer reactions — IB Chemistry SL Revision Notes & Topical Questions

5 min read 12 parts Lesson 1 of 12 1 quiz

📚 IB Chemistry SL Hub / Topical Revision

Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution

pH = -log[H⁺]  |  pOH = -log[OH⁻]  |  pH + pOH = 14  |  [H⁺][OH⁻] = 10⁻¹⁴

Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14

Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.

0%read

Lesson summary

R3.1 Proton transfer reactions — IB CHEMISTRY SL notes: Apply Brønsted-Lowry theory, pH and Kw, compare strong and weak acids and bases, describe neutralisation reactions and interpret pH curves.Acid = a species that donates a proton (H+).Base = a species that accepts a proton us…

Conjugate acid-base pair
Amphiprotic
Water as an acid
Water as a base
Amphoteric
Relationship

Part 1 of 10Brønsted-Lowry acids and bases

Acid

Acid a species that donates a proton (H⁺).

Base

Base a species that accepts a proton using a lone pair of electrons.

Scope

Scope the theory also applies in the gas phase, e.g. NH₃(g) + HCl(g) → NH₄Cl(s).

Hydronium

H⁺ in water is present as H₃O⁺, formed when water accepts a proton using a lone pair, e.g. HCl(g) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq).

Notation

H⁺(aq) and H₃O⁺ are both acceptable, but H₂O must appear in the equation if H₃O⁺ is used.

Proton

Proton an H⁺ ion is a bare proton because hydrogen has lost its only electron.

Part 2 of 10Conjugate pairs and amphiprotic species

Conjugate acid-base pair

Conjugate acid-base pair two species that differ by one H⁺.

Example

Example in CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺, CH₃COOH/CH₃COO⁻ and H₃O⁺/H₂O are the pairs.

Amphiprotic

Amphiprotic able to both donate and accept a proton, e.g. H₂O, HCO₃⁻ and HSO₄⁻.

Water as an acid

Water as an acid it donates a proton to a base such as NH₃, giving NH₄⁺ and OH⁻.

Water as a base

Water as a base it accepts a proton from an acid such as HCl.

Amphoteric

Amphoteric able to react as both an acid and a base, e.g. Al₂O₃ with HCl and with NaOH.

Relationship

Relationship all amphiprotic species are amphoteric, but Al₂O₃ is amphoteric without being amphiprotic.

Part 3 of 10The pH scale

Definition

Definition pH = −log₁₀[H⁺], with no units,

and [H⁺] = 10^−pH.
What the symbols mean: pH has no unit · [H⁺] = concentration of hydrogen ions (mol dm⁻³); each 1 pH unit is a ten-fold change in [H⁺]

Logarithmic scale

Logarithmic scale a decrease of 1 pH unit means a tenfold increase in [H⁺].

Acidic

[H⁺] > 10⁻⁷ mol dm⁻³ and pH < 7 at 298 K.

Basic

[OH⁻] > [H⁺] and pH > 7.

Pure water

Pure water pH 7 at 298 K because [H⁺] = [OH⁻] = 10⁻⁷ mol dm⁻³.

Dilution

Dilution a tenfold dilution of a strong acid raises its pH by 1.

Measuring

Measuring a pH meter is accurate, and universal indicator paper is less accurate and compared with a colour chart.

Part 4 of 10Ion product of water

Equilibrium

H₂O(l) ⇌ H⁺(aq) + OH⁻(aq).

Definition

K_w = [H⁺][OH⁻] = 1.00 × 10⁻¹⁴ at 298 K.

What the symbols mean: K_w = ionic product of water (1.00 × 10⁻¹⁴ mol² dm⁻⁶ at 298 K) · [H⁺], [OH⁻] = ion concentrations (mol dm⁻³)

Use

Use the product of the two concentrations is fixed, so [H⁺] = K_w ÷ [OH⁻].

Effect of temperature

Effect of temperature ionisation of water is endothermic, so K_w rises with temperature, [H⁺] rises and the pH of pure water falls.

Neutrality

Neutrality pure water stays neutral at any temperature because [H⁺] = [OH⁻], and pH 7 is neutral only at 298 K.

Part 5 of 10Strong and weak acids and bases

Property	Strong	Weak
Dissociation	Almost complete	Partial, an equilibrium
Position of equilibrium	Far right	Left
[H⁺] or [OH⁻]	High	Low
Acid examples	HCl, HNO₃, H₂SO₄ (first ionisation)	Ethanoic acid, HCN, H₂S, H₂CO₃
Base examples	Group 1 hydroxides such as NaOH	NH₃, amines, some transition metal hydroxides

1Strength versus concentration

Strength versus concentration strength is the extent of dissociation, while concentration is the amount per dm³ .
- (dilute or concentrated)

2Hydrogen halides

Hydrogen halides acid strength rises HF < HCl < HBr < HI as the longer H–X bond is weaker.

3Sulfuric acid

Sulfuric acid the first ionisation is complete and the second is partial, so 1 mol dm⁻³ H₂SO₄ has less than 2 mol dm⁻³ H⁺.

4Conjugates

Conjugates the stronger the acid, the weaker its conjugate base, e.g. Cl⁻ is a very weak base, and OH⁻ has a weak conjugate acid (water).

Part 6 of 10Distinguishing strong and weak acids

pH at equal concentration the strong acid has the lower pH, e.g. 0.1 mol dm⁻³ HCl is pH 1 and CH₃COOH is about 2.9.

Conductivity

Conductivity the strong acid has more ions, so it conducts better, measured with a conductivity meter.

Reactivity with metals

Reactivity with metals a strong acid reacts faster with Mg, giving more H₂ in a shorter time.

Carbonates

Carbonates both give CO₂, but effervescence is faster with the strong acid.

Oxides and hydroxides

Oxides and hydroxides solids dissolve faster in strong acids, with a larger temperature rise.

Not distinguishable

Not distinguishable the volume of base needed for neutralisation is the same at equal concentration and volume.

Enthalpy

Enthalpy neutralisation by a weak acid is slightly less exothermic because energy is used to dissociate it.

Part 7 of 10Neutralisation and salt formation

Neutralisation and salt formation

Neutralisation

Neutralisation acid + base (alkali) → salt + water, with ionic equation H⁺(aq) + OH⁻(aq) → H₂O(l), which is exothermic.

Strong acid with strong base

ΔH_neut is about −57 kJ mol⁻¹ because the reaction is the same.

Salt names

Salt names hydrochloric acid gives chlorides, nitric acid nitrates and sulfuric acid sulfates.

Acid reacts with	Products
Metal	Salt + hydrogen, e.g. 2HCl + Zn → ZnCl₂ + H₂
Metal oxide	Salt + water, e.g. 2HCl + CaO → CaCl₂ + H₂O
Metal hydroxide	Salt + water, e.g. H₂SO₄ + Mg(OH)₂ → MgSO₄ + 2H₂O
Carbonate	Salt + water + CO₂
Hydrogencarbonate	Salt + water + CO₂, e.g. HCl + NaHCO₃ → NaCl + H₂O + CO₂

Metals not bases
- metals form salts with acids but are not classified as bases because water is not a product.

Part 8 of 10Titration technique

Rinse the burette with the titrant, fill it and record the initial reading.

Pipette a known volume (e.g. 25.00 cm³) of the acid into a conical flask.

Add a few drops of indicator or insert a calibrated pH probe.

Add titrant with swirling, dropwise near the end point, until the indicator permanently changes colour.

Record the burette reading and repeat until titres are concordant within 0.10 cm³.

Average the titres, calculate n(base) = cV, use the mole ratio for n(acid) and then c = n/V.

Part 9 of 10Strong acid-strong base pH curve

Shape

Shape an S-shaped curve of pH against volume of base added.

Start

Start low pH (about 1 for 0.1 mol dm⁻³ HCl) with only strong acid in the flask.

Rise

Rise pH increases slowly as H⁺ is neutralised.

Near equivalence

Near equivalence a very steep rise, about pH 3 to 11.

Equivalence point

Equivalence point the midpoint of the vertical section, pH 7 for strong acid with strong base, with only salt and water present.

After equivalence

After equivalence pH levels off near 13 as excess OH⁻ builds up, and pH = 14 − pOH.

Information

Information the curve gives the initial pH, the pH at equivalence and the volume of base needed.

Part 10 of 10More from the specification

1Weak vs strong base dissociation

Strong bases dissociate completely in aqueous solution, while weak bases like ammonia (NH₃) only partially ionize to establish dynamic equilibrium.

2Electrical conductivity of electrolytes

Measured by ion mobility;
- solutions of strong acids conduct electricity substantially better than equimolar weak acid solutions.

3Carbonate neutralization

Acids react with carbonates to form a metal salt, liquid water, and gaseous carbon dioxide (CO₂), producing vigorous effervescence.

4Aqueous proton transfer

Hydronium ions (H₃O⁺) are formed when an acid donates a proton to water molecules in aqueous solution.

5Aqueous

Dissolved in water.

6Solution

A mixture of a solute dissolved in a solvent.

7Carbon dioxide

A gas that turns limewater cloudy.

8Proton donor

Acid that gives H^{+} ions.

Key terms

Term	Meaning
Brønsted-Lowry acid	A species that donates a proton.
Brønsted-Lowry base	A species that accepts a proton using a lone pair.
Conjugate acid-base pair	Two species that differ by a single H⁺.
Amphiprotic	Able to both donate and accept a proton.
pH	The negative logarithm to base 10 of the hydrogen ion concentration.
K_w	The ion product of water, [H⁺][OH⁻] = 1.00 × 10⁻¹⁴ at 298 K.
Equivalence point	The point where acid and base have reacted in exact stoichiometric amounts.

Avoid these mistakes

Confusing strength with concentration — strength is the degree of dissociation.

Saying pure water is only neutral at pH 7 — it is neutral whenever [H⁺] = [OH⁻].

Distinguishing strong and weak acids by the volume of base needed — it is the same at equal concentration.

Saying a one-unit pH change is a small change — it is a tenfold change in [H⁺].

Last updated: 14 October 2026