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Relative Atomic Mass and Relative Molecular Mass

3.1 Relative Atomic Mass and Relative Molecular Mass
CHEMISTRY • Form 4 • Chapter 3: The Mole Concept, Chemical Formula and Equation

3.1Relative Atomic Mass and Relative Molecular Mass

Atoms and molecules are extremely small and impossible to count or weigh individually. To overcome this difficulty, chemists compare the masses of atoms and molecules to an agreed standard element using the concepts of relative atomic mass (RAM), relative molecular mass (RMM), and relative formula mass (RFM).

Learning Objectives

  • Conceptualise relative atomic mass (RAM) and relative molecular mass (RMM) based on the carbon-12 scale.
  • Explain why carbon-12 was chosen as the international standard atom to determine relative atomic mass.
  • Calculate the relative molecular mass (RMM) of molecular substances by summing up the relative atomic masses of all constituent atoms.
  • Calculate the relative formula mass (RFM) of ionic substances using chemical formulae.

Balance Comparison of Masses on the Carbon-12 Scale

Show how masses of atoms and molecules balance with units representing \(\frac{1}{12}\) of a carbon-12 atom.

Brief Explanation

Understand the key ideas behind relative masses.

Relative atomic mass (RAM) is a comparative value, so it has no unit. Chemists compare the mass of an atom to the mass of a standard atom because individual atoms are too tiny to weigh directly.

1. Historical Standard: Hydrogen

Hydrogen was initially chosen because it is the lightest atom, assigned a mass of 1 unit. One carbon atom is as heavy as 12 hydrogen atoms, giving carbon a relative atomic mass of 12. However, using hydrogen encountered various experimental problems.

2. The Carbon-12 Standard

In 1961, chemists worldwide agreed to adopt the carbon-12 atom as the international standard. One atom of carbon-12 is assigned a definite mass of exactly 12 units.

Reason Explanation
Solid at room temperature Carbon-12 can be handled easily.
Combines easily It combines easily with other elements and is found in most substances.
Major isotope Carbon-12 is the major isotope of carbon with an abundance of 99%, making its relative atomic mass exactly 12.0.

Relative Atomic Mass (RAM)

The average mass of an atom of the element compared to \(\frac{1}{12}\) of the mass of one carbon-12 atom.

\[\text{Relative atomic mass of an element} = \dfrac{\text{Average mass of one atom of the element}}{\frac{1}{12} \times \text{Mass of one carbon-12 atom}}\]

Example: The RAM of helium is 4. This means the average mass of one helium atom is 4 times the mass of \(\frac{1}{12}\) of a carbon-12 atom.

Relative Molecular Mass (RMM)

The average mass of a molecule compared to \(\frac{1}{12}\) of the mass of one carbon-12 atom.

\[\text{Relative molecular mass of a substance} = \dfrac{\text{Average mass of one molecule}}{\frac{1}{12} \times \text{Mass of one carbon-12 atom}}\]

Example: A water molecule (H2O) has an RMM of 18, meaning its mass is 18 times the mass of \(\frac{1}{12}\) of a carbon-12 atom.

Substance Calculation Relative mass
Oxygen, O2 \(2 \times 16\) 32
Water, H2O \(2(1) + 16\) 18
Glucose, C6H12O6 \(6(12) + 12(1) + 6(16) = 72 + 12 + 96\) 180
Relative formula mass (RFM) is used specifically for ionic substances. It is calculated by summing up the relative atomic masses of all atoms shown in the formula unit of the ionic substance. This principle applies because the mass of an ion does not differ significantly from the mass of the neutral atom from which it is formed.

Worked Example: Zinc chloride, ZnCl2

\[\text{RFM} = \text{RAM of Zn} + 2(\text{RAM of Cl})\]

\[= 65 + 2(35.5) = 65 + 71 = 136\]

Worked Example: Aluminium sulphate, Al2(SO4)3

\[\text{RFM} = 2(\text{RAM of Al}) + 3[\text{RAM of S} + 4(\text{RAM of O})]\]

\[= 2(27) + 3[32 + 4(16)] = 54 + 3[96] = 54 + 288 = 342\]

Try These Questions First

Answer mentally, then check the explanation.

1

Why does relative atomic mass (RAM) have no unit?

Answer: Because it is a ratio or comparative value comparing the mass of an atom against a standard mass (\(\frac{1}{12}\) of a carbon-12 atom).
2

One magnesium atom is twice as heavy as one carbon-12 atom. What is the RAM of magnesium?

Answer: Since carbon-12 is 12 units, magnesium is \(2 \times 12 = 24\).
3

Calculate the relative formula mass of zinc chloride, ZnCl2, given that the RAM of Zn = 65 and Cl = 35.5.

Answer: \(\text{RFM} = 65 + 2(35.5) = 65 + 71 = 136\).

Common Mistakes

Avoid these errors when working with relative masses.

!
Writing units (such as grams, g) for relative masses (RAM, RMM, and RFM). Relative masses are dimensionless ratios.
!
Forgetting to multiply subscripts outside brackets in chemical formulae. For example, in Al2(SO4)3, miscounting oxygen atoms as 4 instead of \(3 \times 4 = 12\).
!
Confusing RMM (used for covalent molecular compounds) with RFM (used for ionic compounds).

Common Misconceptions

Compare each misconception with the correct fact.

Misconception

Carbon-12 was chosen because it is the lightest element.

Correct Fact

Hydrogen is the lightest element, but carbon-12 was chosen because it is a solid at room temperature, easily handled, highly abundant (99%), and forms compounds with many other elements.

Misconception

Losing or gaining electrons changes an ion's mass significantly compared to its neutral atom.

Correct Fact

Electrons have negligible mass, so the mass of an ion does not differ significantly from the mass of its parent atom; hence atomic masses are directly summed for RFM.

Summary

Key facts to remember.

  • Relative atomic mass (RAM), relative molecular mass (RMM), and relative formula mass (RFM) compare the masses of atoms, molecules, and ionic formula units to \(\frac{1}{12}\) of the mass of a carbon-12 atom.
  • Relative masses have no units.
  • Carbon-12 is the universal standard, assigned an exact mass of 12 units.
  • RMM is used for covalent molecular compounds, while RFM is used for ionic compounds.
  • Relative masses are determined by adding together the RAM values of all the atoms in a chemical formula.

Quick Activity

Complete all parts before checking your score.

A. Objective Quiz

1

What is the standard used internationally since 1961 to determine relative atomic mass?

2

What is the relative molecular mass (RMM) of carbon dioxide, CO2? [Relative atomic mass: C = 12, O = 16]

B. Fill in the Blanks

3

Because relative atomic mass is a comparative value comparing an atom to a standard, it has no ______.

4

The relative mass of an ionic substance calculated by summing the masses of its constituent ions is called relative ______ mass.

C. Match the Term or Substance with Its Description

Drag each term on desktop. On mobile, tap a term and then tap the matching box.

Term / Substance

Carbon-12 Water (H2O) RFM Helium

Description / Value

1Assigned a definite standard mass of 12 units
2Has a relative molecular mass (RMM) of 18
3Concept of relative mass used specifically for ionic substances
4Has an average atomic mass equal to 4 standard units
Complete every answer before checking.
 

Keywords

Tap or hover over a term to read its meaning.

Relative atomic mass (RAM) Relative molecular mass (RMM) Relative formula mass (RFM) Carbon-12 scale Standard atom
The views expressed in the notes here are those of the contributor and don’t necessarily represent the views of Pandai.

Relative Atomic Mass and Relative Molecular Mass

3.1 Relative Atomic Mass and Relative Molecular Mass
CHEMISTRY • Form 4 • Chapter 3: The Mole Concept, Chemical Formula and Equation

3.1Relative Atomic Mass and Relative Molecular Mass

Atoms and molecules are extremely small and impossible to count or weigh individually. To overcome this difficulty, chemists compare the masses of atoms and molecules to an agreed standard element using the concepts of relative atomic mass (RAM), relative molecular mass (RMM), and relative formula mass (RFM).

Learning Objectives

  • Conceptualise relative atomic mass (RAM) and relative molecular mass (RMM) based on the carbon-12 scale.
  • Explain why carbon-12 was chosen as the international standard atom to determine relative atomic mass.
  • Calculate the relative molecular mass (RMM) of molecular substances by summing up the relative atomic masses of all constituent atoms.
  • Calculate the relative formula mass (RFM) of ionic substances using chemical formulae.

Balance Comparison of Masses on the Carbon-12 Scale

Show how masses of atoms and molecules balance with units representing \(\frac{1}{12}\) of a carbon-12 atom.

Brief Explanation

Understand the key ideas behind relative masses.

Relative atomic mass (RAM) is a comparative value, so it has no unit. Chemists compare the mass of an atom to the mass of a standard atom because individual atoms are too tiny to weigh directly.

1. Historical Standard: Hydrogen

Hydrogen was initially chosen because it is the lightest atom, assigned a mass of 1 unit. One carbon atom is as heavy as 12 hydrogen atoms, giving carbon a relative atomic mass of 12. However, using hydrogen encountered various experimental problems.

2. The Carbon-12 Standard

In 1961, chemists worldwide agreed to adopt the carbon-12 atom as the international standard. One atom of carbon-12 is assigned a definite mass of exactly 12 units.

Reason Explanation
Solid at room temperature Carbon-12 can be handled easily.
Combines easily It combines easily with other elements and is found in most substances.
Major isotope Carbon-12 is the major isotope of carbon with an abundance of 99%, making its relative atomic mass exactly 12.0.

Relative Atomic Mass (RAM)

The average mass of an atom of the element compared to \(\frac{1}{12}\) of the mass of one carbon-12 atom.

\[\text{Relative atomic mass of an element} = \dfrac{\text{Average mass of one atom of the element}}{\frac{1}{12} \times \text{Mass of one carbon-12 atom}}\]

Example: The RAM of helium is 4. This means the average mass of one helium atom is 4 times the mass of \(\frac{1}{12}\) of a carbon-12 atom.

Relative Molecular Mass (RMM)

The average mass of a molecule compared to \(\frac{1}{12}\) of the mass of one carbon-12 atom.

\[\text{Relative molecular mass of a substance} = \dfrac{\text{Average mass of one molecule}}{\frac{1}{12} \times \text{Mass of one carbon-12 atom}}\]

Example: A water molecule (H2O) has an RMM of 18, meaning its mass is 18 times the mass of \(\frac{1}{12}\) of a carbon-12 atom.

Substance Calculation Relative mass
Oxygen, O2 \(2 \times 16\) 32
Water, H2O \(2(1) + 16\) 18
Glucose, C6H12O6 \(6(12) + 12(1) + 6(16) = 72 + 12 + 96\) 180
Relative formula mass (RFM) is used specifically for ionic substances. It is calculated by summing up the relative atomic masses of all atoms shown in the formula unit of the ionic substance. This principle applies because the mass of an ion does not differ significantly from the mass of the neutral atom from which it is formed.

Worked Example: Zinc chloride, ZnCl2

\[\text{RFM} = \text{RAM of Zn} + 2(\text{RAM of Cl})\]

\[= 65 + 2(35.5) = 65 + 71 = 136\]

Worked Example: Aluminium sulphate, Al2(SO4)3

\[\text{RFM} = 2(\text{RAM of Al}) + 3[\text{RAM of S} + 4(\text{RAM of O})]\]

\[= 2(27) + 3[32 + 4(16)] = 54 + 3[96] = 54 + 288 = 342\]

Try These Questions First

Answer mentally, then check the explanation.

1

Why does relative atomic mass (RAM) have no unit?

Answer: Because it is a ratio or comparative value comparing the mass of an atom against a standard mass (\(\frac{1}{12}\) of a carbon-12 atom).
2

One magnesium atom is twice as heavy as one carbon-12 atom. What is the RAM of magnesium?

Answer: Since carbon-12 is 12 units, magnesium is \(2 \times 12 = 24\).
3

Calculate the relative formula mass of zinc chloride, ZnCl2, given that the RAM of Zn = 65 and Cl = 35.5.

Answer: \(\text{RFM} = 65 + 2(35.5) = 65 + 71 = 136\).

Common Mistakes

Avoid these errors when working with relative masses.

!
Writing units (such as grams, g) for relative masses (RAM, RMM, and RFM). Relative masses are dimensionless ratios.
!
Forgetting to multiply subscripts outside brackets in chemical formulae. For example, in Al2(SO4)3, miscounting oxygen atoms as 4 instead of \(3 \times 4 = 12\).
!
Confusing RMM (used for covalent molecular compounds) with RFM (used for ionic compounds).

Common Misconceptions

Compare each misconception with the correct fact.

Misconception

Carbon-12 was chosen because it is the lightest element.

Correct Fact

Hydrogen is the lightest element, but carbon-12 was chosen because it is a solid at room temperature, easily handled, highly abundant (99%), and forms compounds with many other elements.

Misconception

Losing or gaining electrons changes an ion's mass significantly compared to its neutral atom.

Correct Fact

Electrons have negligible mass, so the mass of an ion does not differ significantly from the mass of its parent atom; hence atomic masses are directly summed for RFM.

Summary

Key facts to remember.

  • Relative atomic mass (RAM), relative molecular mass (RMM), and relative formula mass (RFM) compare the masses of atoms, molecules, and ionic formula units to \(\frac{1}{12}\) of the mass of a carbon-12 atom.
  • Relative masses have no units.
  • Carbon-12 is the universal standard, assigned an exact mass of 12 units.
  • RMM is used for covalent molecular compounds, while RFM is used for ionic compounds.
  • Relative masses are determined by adding together the RAM values of all the atoms in a chemical formula.

Quick Activity

Complete all parts before checking your score.

A. Objective Quiz

1

What is the standard used internationally since 1961 to determine relative atomic mass?

2

What is the relative molecular mass (RMM) of carbon dioxide, CO2? [Relative atomic mass: C = 12, O = 16]

B. Fill in the Blanks

3

Because relative atomic mass is a comparative value comparing an atom to a standard, it has no ______.

4

The relative mass of an ionic substance calculated by summing the masses of its constituent ions is called relative ______ mass.

C. Match the Term or Substance with Its Description

Drag each term on desktop. On mobile, tap a term and then tap the matching box.

Term / Substance

Carbon-12 Water (H2O) RFM Helium

Description / Value

1Assigned a definite standard mass of 12 units
2Has a relative molecular mass (RMM) of 18
3Concept of relative mass used specifically for ionic substances
4Has an average atomic mass equal to 4 standard units
Complete every answer before checking.
 

Keywords

Tap or hover over a term to read its meaning.

Relative atomic mass (RAM) Relative molecular mass (RMM) Relative formula mass (RFM) Carbon-12 scale Standard atom
The views expressed in the notes here are those of the contributor and don’t necessarily represent the views of Pandai.
Chapter : Mole Concept, Chemical Formulae and Equations
Topic : Calculating relative molecular mass and relative formula mass
Form 4 Chemistry
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