Which peak in a mass spectrum gives the relative molecular mass of a compound?

The molecular ion peak, which is the peak at the highest mass-to-charge ratio apart from small isotope peaks beyond it. Its m/z value equals the relative molecular mass because the ion carries a single positive charge.

What is the molecular ion?

It is the ion formed when a whole molecule loses one electron, written M⁺ or M⁺•. It has essentially the same mass as the molecule, since the mass of an electron is negligible.

Write an equation for the formation of the molecular ion of methanol.

CH₃OH + e⁻ → CH₃OH⁺ + 2e⁻. A high-energy electron knocks one electron out of the molecule, leaving a positively charged ion.

Why does a mass spectrum contain peaks at masses below that of the molecular ion?

Some molecular ions carry enough energy to break apart into smaller pieces. This fragmentation splits the ion into a positively charged fragment and a neutral radical, and the charged fragment gives its own peak at a lower m/z value.

Write a general equation to represent the fragmentation of a molecular ion.

M⁺ → X⁺ + Y•, where X⁺ is a positively charged fragment ion and Y• is a neutral radical.

Why is the neutral radical produced during fragmentation not seen in the spectrum?

The spectrometer separates and detects ions by their response to electric and magnetic fields. A neutral particle carries no charge, so it is neither accelerated nor deflected and never reaches the detector.

WJEC AS-Level Chemistry 1410QS

Formulae and Equations

Topic overview Charges on common ions Building formulae for ionic and molecular compounds Assigning oxidation numbers Using oxidation numbers to follow redox changes Balancing equations and using state symbols Ionic and half equations

Basic Ideas About Atoms

Topic overview Radioactive decay and nuclear equations Alpha, beta and gamma radiation Half-life, dating and the uses of radioisotopes: half-life of radioactive decay Half-life, dating and the uses of radioisotopes: adverse consequences for living cells Ionisation energies and electronic structure Orbitals and the filling of sub-shells Atomic spectra and the energy of light: atomic emission spectrum of the hydrogen atom Atomic spectra and the energy of light: relationship between energy and frequency Atomic spectra and the energy of light: significance of the frequency

Chemical Calculations

Topic overview Relative masses and the mass spectrometer: relative atomic Relative masses and the mass spectrometer: how a mass spectrometer works Relative masses and the mass spectrometer: molecular ion The mole, the Avogadro constant and molar mass: relative mass terms and simple mass spectra The mole, the Avogadro constant and molar mass: moles and percentage composition by mass Empirical and molecular formulae Concentration, solubility and titration calculations: concentration in mol dm-3 and in g dm-3 Concentration, solubility and titration calculations: stoichiometry and the limiting reagent Concentration, solubility and titration calculations: titration calculations and estimating a titre Gas volumes and the ideal gas equation Yield, atom economy and experimental uncertainty: concepts of atom economy and percentage yield Yield, atom economy and experimental uncertainty: how to estimate the percentage error

Bonding

Topic overview Ionic bonds and covalent bonds: ionic bonding in terms of ion formation Ionic bonds and covalent bonds: covalent bonding and forces Electronegativity, polarity and in-between bonds: concepts of electronegativity and bond polarity Electronegativity, polarity and in-between bonds: forces Forces between molecules Hydrogen bonding Shapes of molecules and ions: VSEPR principle and its use Shapes of molecules and ions: bond angles associated with linear

Solid Structures

Topic overview Ionic crystals: sodium chloride and caesium chloride Giant covalent structures: diamond and graphite Simple molecular solids: iodine and ice Metals and the electron sea Using structure to explain physical properties

The Periodic Table

Topic overview How the Periodic Table is organised Oxidation and reduction as electron transfer Trends across periods and down groups Group 2: reactions, trends and the reasons behind them — reactions with oxygen, water and dilute acid Group 2: reactions, trends and the reasons behind them — trends down the group Group 2: reactions, trends and the reasons behind them — thermal stability of the carbonates Group 2: reactions, trends and the reasons behind them — oxides and hydroxides Identifying ions, making salts and gravimetric analysis: identifying cations with sodium hydroxide Identifying ions, making salts and gravimetric analysis: preparing a soluble salt from an acid Group 7: the halogens — physical properties and the trend in volatility Group 7: the halogens — reactions of the halogens with metals Group 7: the halogens — displacement reactions and the trend Group 7: the halogens — halide ions as reducing agents, uses of chlorine and the chlorate equilibrium

Simple Equilibria and Acid-Base Reactions

Topic overview Reversible reactions and dynamic equilibrium Le Chatelier's principle and shifting the position of equilibrium The equilibrium constant Kc Acids, bases, salts and neutralisation pH, strong acids and weak acids Acid-base titrations and their calculations

Thermochemistry

Topic overview Enthalpy changes and their definitions Calculating enthalpy changes from experimental measurements Determining enthalpy changes in the laboratory Hess's law and energy cycles Average bond enthalpies

Rates of Reaction

Topic overview Collision theory and the conditions that change rate Measuring rate and relating it to concentration Energy profiles and activation energy Temperature and the Boltzmann distribution Catalysts and how they work Following reactions in the laboratory

The Wider Impact of Chemistry

Topic overview Social and economic consequences of chemical manufacture and energy supply Environmental consequences of chemical processes and energy production Green chemistry and sustainable practice

Organic Compounds

Topic overview Drawing and reading organic structures Naming organic compounds Chain length, functional groups and physical properties Structural isomerism Electrophiles, nucleophiles, radicals and bond fission

Hydrocarbons

Topic overview Alkane bonding, combustion and fossil fuels: combustion reaction of alkanes and benefits Alkane bonding, combustion and fossil fuels: combustion calculations and acid rain Radical substitution of alkanes The C=C bond and why alkenes are reactive E–Z isomerism in alkenes Electrophilic addition, Markovnikov's rule and testing for alkenes: mechanism of electrophilic addition Electrophilic addition, Markovnikov's rule and testing for alkenes: Markovnikov's rule and the bromine water test Electrophilic addition, Markovnikov's rule and testing for alkenes: testing for a C=C bond Hydrogenation and addition polymers: conditions required Hydrogenation and addition polymers: nature of addition polymerisation

Halogenoalkanes

Topic overview Nucleophilic substitution by hydroxide ions Elimination to form alkenes Reactivity of the carbon–halogen bond and the silver nitrate test Uses of halogenoalkanes, CFCs and the ozone layer: uses of halogenoalkanes and CFCs Uses of halogenoalkanes, CFCs and the ozone layer: radical mechanism Uses of halogenoalkanes, CFCs and the ozone layer: replacements for CFCs Heating under reflux in the laboratory

Alcohols and Carboxylic Acids

Topic overview Two routes to ethanol: hydration and fermentation — hydration of ethene Two routes to ethanol: hydration and fermentation — fermentation Two routes to ethanol: hydration and fermentation — comparing the two routes on rate Classifying alcohols and dehydrating them Oxidising alcohols to aldehydes, ketones and acids Chemical tests for alcohols and carboxylic acids Carboxylic acids, bases and salts Making an ester and separating it by distillation: esterification Making an ester and separating it by distillation: carrying out the preparation Making an ester and separating it by distillation: separating and purifying Making an ester and separating it by distillation: yield

Instrumental Analysis

Topic overview Reading a mass spectrum Infrared spectra and functional groups NMR spectra and atomic environments Putting the three techniques together

Practical Skills and Techniques

Topic overview Measuring mass, volume, time and temperature Standard solutions and titration technique Indicators and acid–base titration curves Heating, refluxing, distilling and purifying: Heating Heating, refluxing, distilling and purifying: purifying Melting temperature, chromatography and pH measurement Electrochemical cells, rate methods and working safely: Electrochemical cells Electrochemical cells, rate methods and working safely: rate methods and working safely

Planning, Analysis and Evaluation

Topic overview Planning an investigation and controlling variables Recording observations and presenting data Graphs, gradients and processing data Errors, uncertainty, accuracy and precision Evaluating a method and suggesting improvements Research and referencing

Core Experiments and Their Chemistry

Topic overview Gravimetric and acid–base analysis Enthalpy and rate experiments: Enthalpy Enthalpy and rate experiments: rate experiments Redox titrations and electrochemical cells Equilibrium constants and following a titration with a pH probe Organic synthesis, separation and identification: Organic synthesis Organic synthesis, separation and identification: identification tests and chromatography

Mathematical Skills in Chemistry

Topic overview Units, standard form and proportion in chemical calculation: volume, equilibrium, rate and energy units Units, standard form and proportion in chemical calculation: standard form Units, standard form and proportion in chemical calculation: proportion Units, standard form and proportion in chemical calculation: checking answers, buffers, rate constants and units Significant figures, means and uncertainty in experimental data: significant figures in multiplication Significant figures, means and uncertainty in experimental data: relative atomic mass as a weighted mean Significant figures, means and uncertainty in experimental data: absolute and percentage uncertainty Symbols, rearranging equations and logarithms: chemical symbols and what each one asserts Symbols, rearranging equations and logarithms: rearranging an equation Symbols, rearranging equations and logarithms: substituting into equations for Kc, enthalpy and pH Symbols, rearranging equations and logarithms: logarithms, the Arrhenius equation and rearranging formulae Graphs: plotting, gradients, intercepts and tangents — reading a spectrum Graphs: plotting, gradients, intercepts and tangents — reaction order from data and plotting good graphs Graphs: plotting, gradients, intercepts and tangents — Arrhenius plots, zero-order graphs and tangents Graphs: plotting, gradients, intercepts and tangents — initial rates from tangents and interpreting graphs Shape, symmetry and three-dimensional structure: Shape Shape, symmetry and three-dimensional structure: symmetry

Instrumental Analysis

4 subtopics
10 things to know
AS Unit 2

What Instrumental Analysis covers

Instrumental Analysis is examined on AS Unit 2 of WJEC Chemistry, Energy, Rate and Chemistry of Carbon Compounds. 86 recall questions across 4 subtopics.

Where it is assessed

AS Unit 2

Energy, Rate and Chemistry of Carbon Compounds.

Check the WJEC specification ↗

The 4 subtopics

Each one has its own page. WJEC AS Chemistry students sit this topic in the first year.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

Which mass spectrum peak gives the relative molecular mass, how does a molecular ion fragment, and which ions give a peak at m/z 29?

Numbers

The mass spectrum of propanone shows peaks at m/z 58, 43 and 15. Assign each peak.

Key term

The mass spectrum of ethanol shows peaks at m/z 46, 45, 31 and 15. Assign each peak.

Key term

Which bond must a molecule contain if it is to absorb infrared radiation at all?

Numbers

An infrared spectrum shows a broad absorption at 3350 cm⁻¹ and nothing near 1700 cm⁻¹. What functional group is present?

Process

Why is tetramethylsilane the NMR reference, and how many carbon-13 peaks do propanal and benzene give?

Key term

In the carbon-13 spectrum of propanone one peak lies near δ 205 and one near δ 30. Assign them.

Key term

How many proton environments and how many carbon environments does propan-2-ol have?

Key term

A compound has a molecular ion at m/z 60, a broad infrared band at 3350 cm⁻¹, two carbon-13 peaks and three proton peaks. Identify it.

Key term

What determines the wavenumber at which a particular bond absorbs?

Take Instrumental Analysis further

The same content, four other ways.

Commonly confused

Pairs that lose marks when they are blurred together.

The molecular ion peak vs The base peak
The molecular ion peak: the peak from the intact molecule, giving the relative molecular mass. The base peak: the tallest peak, from the most stable fragment.

Questions from this topic

Each one with the model answer underneath.

1
Which peak in a mass spectrum gives the relative molecular mass of a compound?
Model answer
The molecular ion peak, which is the peak at the highest mass-to-charge ratio apart from small isotope peaks beyond it. Its m/z value equals the relative molecular mass because the ion carries a single positive charge.
2
What is the molecular ion?
Model answer
It is the ion formed when a whole molecule loses one electron, written M⁺ or M⁺•. It has essentially the same mass as the molecule, since the mass of an electron is negligible.
3
Write an equation for the formation of the molecular ion of methanol.
Model answer
CH₃OH + e⁻ → CH₃OH⁺ + 2e⁻. A high-energy electron knocks one electron out of the molecule, leaving a positively charged ion.
4
Why does a mass spectrum contain peaks at masses below that of the molecular ion?
Model answer
Some molecular ions carry enough energy to break apart into smaller pieces. This fragmentation splits the ion into a positively charged fragment and a neutral radical, and the charged fragment gives its own peak at a lower m/z value.
5
Write a general equation to represent the fragmentation of a molecular ion.
Model answer
M⁺ → X⁺ + Y•, where X⁺ is a positively charged fragment ion and Y• is a neutral radical.
6
Why is the neutral radical produced during fragmentation not seen in the spectrum?
Model answer
The spectrometer separates and detects ions by their response to electric and magnetic fields. A neutral particle carries no charge, so it is neither accelerated nor deflected and never reaches the detector.

See the rest and revise

What is inside Levo

Six ways to work the same material.

1Closed book

2Open book

3Closed book

The 3-step method, run for you

Answer cold, learn it, then prove it stuck. Levo tracks the jump between your first attempt and your last.

Formulae and Equations

Basic Ideas About Atoms

Chemical Calculations

Bonding

Cram a whole paper in one sitting

Sort every topic into what you know and what you do not, then drill only the weak ones.

An element is zero.

uncombinedionisedoxidised

Blank Blitz makes facts automatic

Rapid timed gap-fills on the terms and numbers you need to recall without thinking.

AVO₂⁺ + 2H⁺ + e⁻ → VO²⁺ + H₂O.

BVanadium falls from +5 to +4.

COne electron is gained.

Build It gets the exam shape right

Assemble a full-mark answer from its marking points, in the order an examiner wants them.

A gas syringe measures all the gas produced.

TrueFalse

Test It catches misconceptions

Spot the fault in a statement. The traps are the mistakes students actually make.

721941163128510

Brain Breaks between sessions

Short focus games to reset attention, so the next set lands better than the last.

Instrumental Analysis revision questions

Is Instrumental Analysis examined at AS in WJEC Chemistry?: Yes. It sits in AS Unit 2, which you take in the first year, so it is on the AS paper as well as counting towards the full A level.
What does Instrumental Analysis cover in WJEC AS-Level Chemistry?: 4 subtopics: reading a mass spectrum, infrared spectra and functional groups, nmr spectra and atomic environments and putting the three techniques together. Each one has its own page with questions and model answers.
How is Instrumental Analysis organised?: By the WJEC 1410QS specification, subtopic by subtopic, so it matches the order you are taught it in.

Every question in Instrumental Analysis, on a schedule.

Attempt a question from memory, write the correct version, then attempt it again. Levo records the jump between your first attempt and your last, and brings each question back on the day you are due to forget it.

Start revising, £5 a month See how the method works

Or £46 a year, about £3.83 a month. Cancel anytime.