What does a high-resolution mass spectrometer measure that a low-resolution one does not?

The m/z of the molecular ion to four decimal places, giving an accurate relative molecular mass rather than a whole number.

Why can high-resolution mass spectrometry tell apart molecules with the same whole-number Mᵣ?

Relative isotopic masses other than carbon-12 are not exact whole numbers, so different molecular formulae with the same nominal Mᵣ have slightly different accurate masses.

Which isotope has a relative isotopic mass of exactly 12.0000, and why?

Carbon-12, because it is the standard against which all relative atomic and isotopic masses are defined.

Calculate the accurate Mᵣ of ethene, C₂H₄ (¹H = 1.0078, ¹²C = 12.0000).

Mᵣ = 2(12.0000) + 4(1.0078) = 28.0312.

Calculate the accurate Mᵣ of carbon monoxide, CO (¹²C = 12.0000, ¹⁶O = 15.9949).

Mᵣ = 12.0000 + 15.9949 = 27.9949.

Edexcel A-Level Chemistry 9CH0

Atomic Structure and the Periodic Table

Topic overview Protons, neutrons, electrons and isotopes Relative atomic mass and the mass spectrum Ionisation energy and the evidence for sub-shells Orbitals and electron configuration Periodicity across a period and down a group

Bonding and Structure

Topic overview Ionic bonding Covalent bonding, dative bonding and the shapes of molecules Electronegativity, polarity and intermolecular forces Structure and properties

Redox I

Topic overview Oxidation numbers Oxidation, reduction, disproportionation and half-equations

Inorganic Chemistry and the Periodic Table

Topic overview Flame tests and how they work Thermal decomposition of nitrates and carbonates Carrying out flame tests and testing the gases Group 2 elements and their compounds Trends in Group 7 and predicting fluorine and astatine Halogen displacement reactions Disproportionation and the reactions of chlorine Halides with concentrated sulfuric acid, and the hydrogen halides Testing for halide ions with silver nitrate Testing for carbonate and hydrogencarbonate ions Testing for sulfate and ammonium ions

Formulae, Equations and Amounts of Substance

Topic overview The mole, molar mass and empirical formula The ideal gas equation and molar volume Equations, concentration and reacting quantities Standard solutions, titration and uncertainty Percentage yield, atom economy and acid reactions Core practicals 1 to 3 Empirical formula and water of crystallisation by experiment Apparatus, uncertainty and mathematical skills

Organic Chemistry I

Topic overview Formulae, homologous series and functional groups IUPAC nomenclature Types of organic reaction and structural isomerism Stereoisomerism and E/Z nomenclature Practical techniques in organic chemistry Alkanes, crude oil and combustion pollutants Radical substitution Alkenes, the double bond and hydrogenation Electrophilic addition, Markovnikov's rule and the test for an alkene Addition polymers and waste polymers Nucleophilic substitution of halogenoalkanes Rates of hydrolysis and the SN1 and SN2 mechanisms Classifying and oxidising alcohols Alcohols: practical techniques and the core practicals

Modern Analytical Techniques I

Topic overview Mass spectrometry Infrared spectroscopy and the characteristic absorptions Deducing functional groups from an infrared spectrum

Energetics I

Topic overview Enthalpy changes, standard definitions and calorimetry Hess's law and bond enthalpies

Kinetics I

Topic overview Collision theory and factors affecting rate Maxwell-Boltzmann distribution and temperature Catalysts, reaction profiles and measuring rate

Equilibrium I

Topic overview Dynamic equilibrium and Le Chatelier's principle The equilibrium constant Kc

Equilibrium II

Topic overview Partial pressures and Kp Calculating Kp and the effect of conditions

Acid-Base Equilibria

Topic overview Bronsted-Lowry acids and bases, and the pH scale Strong and weak acids, Ka and Kw Calculating pH, and titration curves Buffer solutions and the Henderson-Hasselbalch equation Buffers in blood, and the core practical

Energetics II

Topic overview Lattice energy Born-Haber cycles and electron affinity Entropy and the direction of change Gibbs free energy and thermodynamic feasibility

Redox II

Topic overview Standard electrode potentials and the hydrogen electrode Electrochemical cells, emf and cell diagrams Feasibility, the electrochemical series and disproportionation Storage cells, fuel cells and redox titrations

Transition Metals

Topic overview Electron configuration and what makes a transition metal Complex ions, ligands and the colour of transition metal ions Coordination number, monodentate ligands and the shapes of complexes Bidentate and multidentate ligands, and ligand exchange Vanadium and chromium chemistry Reactions with sodium hydroxide and ammonia Cobalt and copper complexes Ligand exchange, amphoteric behaviour and the chelate effect Transition metals as catalysts

Kinetics II

Topic overview Rate equations, orders and the rate constant Determining orders from experiments Rate-determining step and mechanisms Arrhenius equation and activation energy

Organic Chemistry II

Topic overview Chirality Aldehydes, ketones and their oxidation Reduction and nucleophilic addition of HCN Testing for a carbonyl group: 2,4-DNPH and the iodoform reaction Carboxylic acids and their reactions Esters and acyl chlorides Reflux, extraction and recrystallisation Distillation, melting temperature and working safely

Organic Chemistry III

Topic overview The structure of benzene and the evidence for it Electrophilic substitution of benzene Nitration, halogenation, Friedel-Crafts and phenol Amines: structure and basicity Reactions of amines and preparing them Amides and condensation polymers Amino acids, zwitterions and proteins Empirical and molecular formulae from analytical data Grignard reagents and multi-step synthesis Purification techniques and the aspirin preparation

Modern Analytical Techniques II

Topic overview Carbon-13 and proton NMR Splitting, integration and the reference standard Chromatography High-resolution mass spectrometry

Modern Analytical Techniques II

High-resolution mass spectrometry

3 things to know
Papers 2 and 3

Overview

Accurate relative molecular masses to four decimal places, why carbon-12 is exactly 12.0000, calculating accurate masses from isotopic masses, telling apart formulae with the same whole-number Mr such as CO, N2 and C2H4, and why an accurate mass alone cannot identify a structure.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Process

Which of ethanol and methanoic acid has an accurate Mᵣ of 46.0417 (¹H = 1.0078, ¹²C = 12.0000, ¹⁶O = 15.9949)?

Process

Why can an accurate relative molecular mass not identify a compound on its own?

Process

Calculate the accurate Mᵣ of nitrogen, N₂ (¹⁴N = 14.0031).

Questions from this subtopic

Each one with the model answer underneath.

1
What does a high-resolution mass spectrometer measure that a low-resolution one does not?
Model answer
The m/z of the molecular ion to four decimal places, giving an accurate relative molecular mass rather than a whole number.
2
Why can high-resolution mass spectrometry tell apart molecules with the same whole-number Mᵣ?
Model answer
Relative isotopic masses other than carbon-12 are not exact whole numbers, so different molecular formulae with the same nominal Mᵣ have slightly different accurate masses.
3
Which isotope has a relative isotopic mass of exactly 12.0000, and why?
Model answer
Carbon-12, because it is the standard against which all relative atomic and isotopic masses are defined.
4
Calculate the accurate Mᵣ of ethene, C₂H₄ (¹H = 1.0078, ¹²C = 12.0000).
Model answer
Mᵣ = 2(12.0000) + 4(1.0078) = 28.0312.
5
Calculate the accurate Mᵣ of carbon monoxide, CO (¹²C = 12.0000, ¹⁶O = 15.9949).
Model answer
Mᵣ = 12.0000 + 15.9949 = 27.9949.
6
Calculate the accurate Mᵣ of nitrogen, N₂ (¹⁴N = 14.0031).
Model answer
Mᵣ = 2(14.0031) = 28.0062.

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Atomic Structure and the Periodic Table

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Redox I

Inorganic Chemistry and the Periodic Table

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