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]

5.7Unit 5. Transition Metals and Organic Nitrogen Chemistry

Organic Synthesis — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

15 min read 29 parts Lesson 6 of 43 1 quiz

📚 IAL Edexcel Linear (Chemistry) 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.

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Lesson summary

5.7 Organic Synthesis — Overview, Structure Deduction and Chain Lengthening — IAL EDEXCEL LINEAR (CHEMISTRY) notes: Deduce organic structures from reaction and spectral data, plan multi-step routes and lengthen carbon chains.Step 1 — find the empirical formula from combustion or …

Infrared spectroscopy
Mass spectrometry
Target and starting molecule
Functional group changes
Retrosynthesis
Number of steps

Part 1 of 9Order of structure deduction

1Step 1

Step 1 find the empirical formula from combustion or percentage composition data.

2Step 2

Step 2 find the molecular formula using the relative molecular mass from mass spectrometry.

3Step 3

Step 3 identify the functional groups present from test-tube reactions and infrared spectra.

4Step 4

Step 4 deduce the overall structure from mass spectrum fragments, 13C NMR and 1H NMR.

Part 2 of 9Combustion analysis

Method

Method a known mass of compound is burned in excess dry oxygen and the CO₂ and H₂O produced are weighed.

Mass of C

Mass of C equals 12.0/44.0 × mass of CO₂ formed.
Mass of H — equals 2.0/18.0 × mass of H₂O formed (two H atoms per water molecule).
Mass of O — found by difference, as 100% minus %C minus %H.

Empirical formula

Empirical formula obtained by dividing each percentage by the relative atomic mass and then by the smallest ratio.

Part 3 of 9Test-tube reactions for functional groups

Reagent	Group detected	Positive result
Bromine water	C=C	Orange solution decolourises
Acidified potassium dichromate(VI)	Primary or secondary alcohol (or aldehyde)	Orange to green
Tollens' reagent	Aldehyde	Silver mirror
Fehling's solution	Aldehyde	Blue to red-brown precipitate
2,4-DNPH	C=O in aldehydes and ketones	Orange precipitate
Sodium carbonate or NaHCO₃	Carboxylic acid	Effervescence of CO₂
Iodine with NaOH (iodoform)	CH₃C=O or CH₃CH(OH) group	Pale yellow precipitate of CHI₃
PCl₅	–OH group	Steamy fumes of HCl

Part 4 of 9Spectral evidence

Infrared spectroscopy

Infrared spectroscopy identifies functional groups from characteristic bond absorptions.

Mass spectrometry

Mass spectrometry gives the molecular ion peak for the molecular formula and fragment peaks for the structure.

13C NMR

13C NMR shows the number of different carbon environments in the molecule.

1H NMR

1H NMR shows the number of proton environments, their ratios from the integration and the neighbouring protons from splitting.

Part 5 of 9Planning a synthetic route

1Target and starting molecule

Target and starting molecule draw both and compare the number of carbon atoms.

2Functional group changes

Functional group changes list each change and match it to a known reaction with its reagents and conditions.

3Retrosynthesis

Retrosynthesis work backwards from the target to compounds that can be made from the starting material.

4Number of steps

Number of steps keep the route as short as possible, because the overall yield falls with every step.

5Hazards

Hazards choose control measures from the hazard data of the reagents.

Part 6 of 9Lengthening the carbon chain

Method	Reagents and conditions	Product	Carbons added
Halogenoalkane + cyanide	KCN in ethanol, heat under reflux	Nitrile	1
Carbonyl + HCN	HCN or KCN with acid, buffered	Hydroxynitrile	1
Grignard + CO₂	RMgX in dry ether, then dilute acid	Carboxylic acid	R group + 1
Grignard + carbonyl	RMgX in dry ether, then dilute acid	Alcohol	R group

1Nitrile hydrolysis

RCN + 2H₂O + H⁺ → RCOOH + NH₄⁺ on heating with dilute acid.

2Nitrile reduction

RCN + 4[H] → RCH₂NH₂ using LiAlH₄ in dry ether.

Part 7 of 9Grignard reagents

Preparation

Preparation a halogenoalkane reacts with magnesium in dry ether to give RMgX.

Conditions

Conditions dry apparatus is essential because water protonates the reagent to an alkane.

Methanal

Methanal gives a primary alcohol.

Other aldehydes

Other aldehydes give secondary alcohols.

Ketones

Ketones give tertiary alcohols.

Carbon dioxide

Carbon dioxide gives a carboxylic acid.

Part 8 of 9Preparation and purification overview

Preparation

Preparation heat under reflux to complete a slow reaction without loss, or distil off a product as it forms.

Liquids

Liquids wash in a separating funnel, dry with an anhydrous salt, filter or decant, then redistil at the expected boiling point.

Solids

Solids recrystallise from the minimum hot solvent and filter under reduced pressure.

Purity

Purity a sharp melting point matching the data book value, a single TLC spot and no extra peaks in spectra.

Part 9 of 9More from the specification

Nucleophilic substitution

Nucleophilic substitution a nucleophile replaces the halogen in a haloalkane, for example hydroxide forming an alcohol

Distillation

Distillation separates a product of lower boiling point from the mixture, with the thermometer at the vapour temperature and a condenser to cool it

Separating a layer

Separating a layer a separating funnel splits the organic layer from the aqueous one, followed by drying with anhydrous salt

Analysis of the sample

Analysis of the sample boiling point, melting point and spectra show purity, and reflux keeps the mixture at a constant temperature

Electrophilic substitution

Replacement of H on the ring by an electrophile.

Nucleophilic substitution

Replacement of an atom or group by a nucleophile.

Simple distillation

Separating a solvent from a solution by evaporating and condensing it.

Drying agent

An anhydrous salt that removes water from an organic liquid.

Distillation

Separating a liquid from a solution by boiling and condensing the vapour.

Substitution

One atom or group is replaced by another.

Condenser

Apparatus that cools gas back to liquid.

Temperature

A measure of the average kinetic energy of the particles.

Mixture

Two or more substances mixed but not chemically combined.

Combustion analysis

Burning a known mass of compound in excess oxygen and weighing the CO₂ and H₂O formed.

Key terms

Term	Meaning
Synthetic route	A sequence of reactions converting a starting material into a target molecule.
Retrosynthesis	Working backwards from the target molecule to suitable starting materials and steps.
Nitrile	A compound containing the C≡N group, used to add one carbon to a chain.
Hydroxynitrile	A compound with both OH and CN groups, formed when HCN adds to an aldehyde or ketone.
Grignard reagent	An organomagnesium halide RMgX made from a halogenoalkane and magnesium in dry ether.
Empirical formula	The simplest whole-number ratio of atoms of each element in a compound.
Percentage yield	Actual yield divided by theoretical yield × 100.

Avoid these mistakes

Using bromine water to identify an alcohol — it only detects C=C, so use acidified dichromate for alcohols.

Using water as the solvent for a Grignard reagent — water destroys it, so dry ether must be used.

Stopping nitrile hydrolysis at the amide — heating with dilute acid gives the carboxylic acid.

Forgetting the extra carbon from cyanide — each KCN substitution lengthens the chain by one carbon.

Last updated: 15 October 2026