3.2.2

Relative Molecular Mass (Mr)

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Understanding Relative Molecular Mass (Mr)

Relative Molecular Mass (Mr) is the sum of relative atomic masses in a molecule. For ionic compounds, it's Relative Formula Mass. Calculate by adding atomic masses from the Periodic Table.

What is relative molecular mass?

What is relative molecular mass?

  • Relative molecular mass (Mr) is the sum of the relative atomic masses of the atoms in a molecule.
    • It shows how heavy one molecule is compared to one-twelfth of a carbon-12 atom.
  • Chemists use Mr to compare molecule sizes.
What is relative formula mass?

What is relative formula mass?

  • Relative formula mass (Mr) is used for ionic compounds.
    • Ionic compounds do not form molecules but have formulas.
  • Mr adds up the relative atomic masses of all atoms in the formula.
Using atomic mass from the periodic table

Using atomic mass from the periodic table

  • Each element's relative atomic mass is on the Periodic Table.
  • To find the Mr of a molecule, add the atomic masses of all atoms.
    • For example, water (H2O) has Mr = 2(H) + 1(O) = 2 x 1 + 16 = 18.
Summary of calculations

Summary of calculations

  • To calculate Mr, write the chemical formula.
    • Find atomic masses from the Periodic Table.
    • Add atomic masses, considering the number of atoms.
  • Use Mr for molecules and relative formula mass for ionic compounds.
Jump to other topics
1

States of Matter

1.1

Solids, Liquids, & Gases

1.1.1States of Matter - Solids, Liquids & Gases1.1.2Structure of Solids, Liquids & Gases1.1.3Changes of State1.1.4The Effects of Temperature & Pressure1.1.5Kinetic Theory1.1.6Kinetic Theory - Effects of Temperature & Pressure
1.2

Diffusion

1.2.1Diffusion1.2.2The Effect of Relative Molecular Mass
2

Atoms, Elements & Compounds

2.1

Elements, Compounds & Mixtures

2.1.1Differences Between Elements, Compounds & Mixtures
2.2

Atomic Structure & the Periodic Table

2.2.1The Structure of the Atom2.2.2Relative Charge, Mass & Atomic Numbers2.2.3Electronic Configuration of Elements
2.3

Isotopes

2.3.1Isotopes2.3.2Electronic Configuration of Isotopes2.3.3Calculating Relative Atomic Mass
2.4

Ions & Ionic Bonds

2.4.1Properties of Ionic Compounds - Structure & Bond2.4.2Formation of Ions & Ionic Bonds2.4.3Formation of Ionic Bonds - Metals & Non-Metals2.4.4Ionic Bonds2.4.5Ionic Compounds2.4.6Giant Lattice Structure of Ionic Compounds
2.5

Simple Molecules & Covalent Bonds

2.5.1Covalent Bonds2.5.2Formation of Covalent Bonds2.5.3Structure & Properties of Simple Molecules2.5.4Covalent Bonds in Simple Molecules2.5.5Properties of Simple Molecular Compounds
2.6

Giant Covalent Structures

2.6.1Giant Covalent Structures - Graphite2.6.2Giant Covalent Structures - Diamond2.6.3Uses of Graphite & Diamonds2.6.4Giant Covalent Structures - Silicon Oxide2.6.5Similarity Between Diamond & Silicon Oxide
2.7

Metallic Bonding

2.7.1Metallic Bonding2.7.2The Properties of Metals
3

Stoichometry

3.1

Formulae

3.1.1Understanding Formulae3.1.2Empirical Formula3.1.3Stoichiometry - Formulae, Word & Symbol Equations3.1.4Deduce & Write Ionic Compound Formulas
3.2

Relative Masses of Atoms & Molecules

3.2.1Relative Atomic Mass3.2.2Relative Molecular Mass (Mr)3.2.3Calculating Reacting Masses
3.3

The Mole & the Avogadro Constant

3.3.1Introducing Moles3.3.2Amounts of Substances3.3.3Moles & Equations3.3.4Calculations Involving Gases3.3.5Calculating Stoichiometric Reacting Masses3.3.6Calculating the Moles of Solute3.3.7Calculating Empirical Formula3.3.8Chemical Equations3.3.9Percentage Yield3.3.10Calculating Yield3.3.11Percentage Purity
4

Electrochemistry

4.1

Electrolysis

4.1.1Electrolytic Process4.1.2Electrolysis Examples4.1.3Electrolysis of a Binary Compound in Molten State4.1.4Electroplating4.1.5Transfer of Charge During Electrolysis4.1.6Electrolysis of Aqueous Solutions4.1.7Electrolysis of Copper Sulfate4.1.8Electrolysis of Halide Compounds4.1.9Half-Equations4.1.10Combustion of Hydrocarbons
4.2

Hydrogen–Oxygen Fuel Cells

4.2.1Fuel Cells4.2.2Hydrogen Fuel Cells4.2.3Comparing Hydrogen-Oxygen Fuel Cells & Petrol
5

Chemical Energetics

5.1

Exothermic & Endothermic Reactions

5.1.1Energy Conservation5.1.2Exothermic Reactions5.1.3Endothermic Reactions5.1.4Energy Change of a Reaction5.1.5Reaction Pathway Diagrams5.1.6Calculating Enthalpy Change
6

Chemical Reactions

6.1

Physical & Chemical Changes

6.1.1Difference btwn Physical & Chemical Changes
6.2

Rate of Reaction

6.2.1Collision Theory & Rate of Reaction6.2.2Factors Affecting Rate of Reaction6.2.3Investigating Rate of Reaction by Change in Mass6.2.4Investigating Rate - Formation of a Gas6.2.5Evaluating Methods to Investigate Reaction Rates
6.3

Reversible Reactions & Equilibrium

6.3.1Reversible Reactions6.3.2Conditions & Equilibrium6.3.3Dynamic Equilibrium6.3.4Changing Conditions - Heat & Water Effects6.3.5Factors Affecting Equilibria - Temperature6.3.6Factors Affecting Equilibria - Pressure6.3.7Factors Affecting Equilibria - Concentration6.3.8The Haber Process6.3.9The Haber Process Equation
6.4

Redox

6.4.1Redox Reactions6.4.2Redox Reactions - OIL RIG6.4.3Examples of Redox Reactions6.4.4Identifying Redox Reactions6.4.5Identifying Redox Reactions by Colour Changes6.4.6Testing for Oxidising & Reducing Agents
7

Acids, Bases & Salts

7.1

The Characteristic Properties of Acids & Bases

7.1.1Acids & Alkali7.1.2Reactions of Metals with Acids7.1.3Reactions of Bases with Acids7.1.4Reactions of Carbonates with Acids7.1.5Identifying Acids7.1.6Properties & Effects of Acids7.1.7Properties of Bases - Reactions with Acids & Salts7.1.8Properties of Alkalis & Indicators7.1.9Strong vs Weak Acids7.1.10Strong vs Weak Bases7.1.11Understanding Universal Indicator Paper7.1.12Acids, Bases & the Neutralisation Reaction
7.2

Oxides

7.2.1Oxides7.2.2Amphoteric Oxides - Reacting with Acids & Bases
7.3

Preparation of Salts

7.3.1Soluble Salts7.3.2Preparing Soluble Salts7.3.3Solubility of Salts7.3.4Preparing Insoluble Salts & Crystallisation
8

The Periodic Table

8.1

Arrangement of Elements

8.1.1The Periodic Table8.1.2Metals & Non-Metals
8.2

Group I Properties

8.2.1Alkali Metals8.2.2Alkali Metals - Properties8.2.3Reactivity of Alkali Metals8.2.4Alkali Metals & Water
8.3

Group VII Properties

8.3.1The Halogens - Properties8.3.2The Halogens - Colours & States8.3.3The Halogen - Displacement 18.3.4The Halogen - Displacement 2
8.4

Transition Elements

8.4.1Transition Metals8.4.2Transition Metals - Special Properties
8.5

Noble Gases

8.5.1Noble Gases - Properties
9

Metals

9.1

Properties of Metals

9.1.1Physical Properties of Metals9.1.2Chemical Properties of Metals9.1.3Metal Oxides
9.2

Uses of Metals

9.2.1Uses of Metals: Aluminium & Copper
9.3

Alloys & Their Properties

9.3.1Alloys9.3.2Alloys 29.3.3Alloys & their Uses in Everyday Life9.3.4Structural Diagrams of Alloys9.3.5Structure of Alloys & Their Strength
9.4

Reactivity Series

9.4.1Metals - Reactivity Series & Reactivity9.4.2Order of Reactivity9.4.3Reactivity of Metals
9.5

Corrosion of Metals

9.5.1Corrosion9.5.2Preventing Corrosion
9.6

Extraction of Metals

9.6.1Extraction of Metals9.6.2Extracting Iron9.6.3Extracting Aluminium from Bauxite
10

Chemistry of the Environment

10.1

Water

10.1.1Testing & Treating Water10.1.2Testing Water Purity Using Melting Point10.1.3Water from Natural Sources10.1.4Potable Water
10.2

Fertilisers

10.2.1Nitrogen & Fertilisers
10.3

Air Quality & Climate

10.3.1Gases in the Atmosphere10.3.2Common Atmospheric Pollutants10.3.3Consequences of Atmospheric Pollutants10.3.4Strategies to Reduce Pollution Effects10.3.5Photochemical Reactions10.3.6Car Engines, Nitrogen Oxides, & Converters
11

Organic Chemistry

11.1

Formulae, Functional Groups & Terminology

11.1.1Formula of a Molecule11.1.2General Formulae of Organic Compounds11.1.3Functional Groups11.1.4Homologous Series11.1.5Saturated & Unsaturated Organic Compounds11.1.6Structural Formula11.1.7Structural Isomers
11.2

Naming Organic Compounds

11.2.1Drawing Displayed Formulae11.2.2Naming Organic Compounds - Unbranched11.2.3Naming Unbranched Esters from Alcohols & Acids11.2.4Naming Structural Formulae
11.3

Fuels

11.3.1Fossil Fuels11.3.2Methane as the Main Constituent of Natural Gas11.3.3Fuels & Hydrocarbons11.3.4Petroleum - A Mixture of Hydrocarbons11.3.5Fractional Distillation11.3.6Fractions11.3.7Uses of Fractions - Fuels
11.4

Alkanes

11.4.1Alkanes11.4.2Properties of Alkanes11.4.3Substitution Reactions of Alkanes with Chlorine
11.5

Alkenes

11.5.1Alkenes11.5.2Shortest Alkenes11.5.3Reactions of Alkenes11.5.4Types of Alkene Reactions11.5.5Cracking11.5.6Testing Saturated & Unsaturated Hydrocarbons11.5.7Properties of Alkenes - Addition Reactions
11.6

Alcohols

11.6.1Alcohols11.6.2Smallest Alcohols11.6.3Fermentation11.6.4Steam11.6.5Fermentation vs Hydration of Ethene11.6.6Manufacture of Ethanol: Fermentation & Hydration11.6.7Combustion of Ethanol11.6.8Uses of Ethanol - Solvent & Fuel11.6.9Advantages & Disadvantages of Ethanol Manufacture
11.7

Carboxylic Acids

11.7.1Carboxylic Acids11.7.2Smallest Carboxylic Acids11.7.3Reactions of Carboxylic Acids11.7.4Formation of Ethanoic Acid
11.8

Polymers

11.8.1Polymers11.8.2Problems With Polymers11.8.3Polymers & Plastics11.8.4Problems with Plastics11.8.5Condensation Polymerisation11.8.6Condensation Polymerisation Examples11.8.7Addition Polymerisation11.8.8Addition Polymerisation Examples11.8.9Structure of Polymers11.8.10Natural Polyamides - Proteins & Amino Acids
12

Experimental Techniques & Chemical Analysis

12.1

Experimental Design

12.1.1Measurement Tools12.1.2Mixtures12.1.3Understanding Solvents, Solutes, & Solutions12.1.4Separating Mixtures12.1.5Distillation12.1.6Advantages & Disadvantages of Experimental Methods
12.2

Acid-base Titrations

12.2.1Acid-base Titrations
12.3

Chromatography

12.3.1Chromatography12.3.2Paper Chromatography12.3.3Chromatogram
12.4

Separation & Purification

12.4.1Purity12.4.2Methods of Separation & Purification
12.5

Identification of Ions & Gases

12.5.1Testing Aqueous Cations 112.5.2Testing Aqueous Cations 212.5.3Testing Cations - Flame Tests12.5.4Testing Cations12.5.5Testing for Aqueous Cations12.5.6Testing Anions - Sulfates12.5.7Testing Anions - Nitrates12.5.8Testing Gases - Hydrogen12.5.9Testing Gases - Oxygen12.5.10Testing Gases - Carbon Dioxide12.5.11Testing Gases - Chlorine12.5.12Identifying Gases

Practice questions on Relative Molecular Mass (Mr)

Can you answer these? Test yourself with free interactive practice on Seneca — used by over 10 million students.

  1. 1
    What is Relative Molecular Mass (Mr) defined as?Multiple choice
  2. 2
    If a molecule has the formula XY<sub>2</sub>, how would you calculate its Mr?Multiple choice
  3. 3
    Calculating Mr:Fill in the list
  4. 4
    A group of scientists is studying a newly discovered molecular compound with the formula C<sub>3</sub>H<sub>6</sub>O. Using the Periodic Table values: C = 12, H = 1, and O = 16, which calculation correctly shows the relative molecular mass (Mr) for this compound?Multiple choice
  5. 5
    In a laboratory, students analyse an ionic compound with the formula NaCl. What is the correct interpretation of its relative formula mass (Mr)?Multiple choice
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Relative Atomic Mass

Calculating Reacting Masses