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Relative Molecular Mass Calculations Chemistry Tutorial

Key Concepts

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Relative Molecular Mass Concepts

The diagram on the right shows a box containing 3 balls:

  • one red ball shown as o
  • two black balls shown as o
o       o
    o    

If a red ball has a mass of 16 g and a black ball has a mass of 1 g, then the mass of all three balls in the box is 16 + 1 + 1 = 18 g

We could write a mathematical expression to find the total mass of the balls in the box:

mass(total) = mass(red ball) + 2 × mass(black ball)

mass(total) = 16 + 2 × 1 = 16 + 2 = 18 g

Chemists often think of atoms as really tiny balls, and refer to this as the particle theory of matter.

If each ball in the box represents an atom making up a water molecule, H2O, then the diagram below shows a box containing a molecule of water in which

  • the red ball o is an oxygen atom
  • each black ball o is hydrogen atom
  • each diagonal line ( \ and / ) represents a chemical bond between an oxygen atom and a hydrogen atom
o       o
  \   /  
    o    

If an oxygen atom has a mass of 16 and a hydrogen atom has a mass of 1, then the mass of all three atoms in the water molecule is 16 + 1 + 1 = 18

We could write a mathematical expression to find the total mass of all the atoms in a water molecule:

mass(water molecule) = mass(oxygen atom) + 2 × mass(hydrogen atom)

mass(water molecule) = 16 + 2 × 1 = 16 + 2 = 18

This is exactly what we are doing when we calculate the relative molecular mass of a compound:

  1. Use the chemical formula to determine how many atoms of each element are present in the compound.

    compund XaYb contains:

    a atoms of element X

    b atoms of element Y

  2. Write a mathematical expression to calculate the total mass of all the elements in the compound (called the relative molecular mass of the compound)

    Mr(XaYb) = relative molecular mass of compound XaYb

    Mr(X) = relative atomic mass of element X

    Mr(Y) = relative atomic mass of element Y

    a = number of atoms of element X

    b = number of atoms of element Y

    Mr(XaYb) = a × Mr(X) + b × Mr(Y)

  3. Use the Periodic Table to find the relative atomic mass (atomic weight) of each element

    Mr(X) = relative atomic mass of element X = some number

    Mr(Y) = relative atomic mass of element Y = another number

  4. Substitute the values for the relative atomic mass (atomic weight) of each element into the equation and then solve the equation to find the relative molecular mass of the compund

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Worked Examples of Relative Molecular Mass Calculations

Calculating the Relative Molecular Mass of a Diatomic Molecule

Carbon monoxide is a diatomic molecule, a molecule made up of two atoms: an atom of carbon (C) and an atom of oxygen (O).
Carbon monoxide has the molecular formula CO

Calculate the relative molecular mass (Mr) of the compound carbon monoxide, CO

Calculating the Relative Molecular Mass of a Triatomic Molecule

Carbon dioxide is a triatomic molecule, a molecule made up of 3 atoms.
Carbon dioxide has the molecular formula CO2
Each molecule of carbon dioxide is made up of 1 atom of carbon (C) and 2 atoms of oxygen (O).

Calculate the relative molecular mass (Mr) of the compound carbon dioxide, CO2.

Calculating the Relative Molecular Mass of a Polyatomic Compound

Calcium hydroxide is a polyatomic compound, a compound made up of many "atoms".
Calcium hydroxide has the formula Ca(OH)2
The compound is made up of calcium ions, Ca2+, and hydroxide ions, OH-
For every 1 ion of calcium there are 2 hydroxide ions.
Each hydroxide ion, OH-, is made up of 1 atom of oxygen (O) and 1 atom of hydrogen (H).

Calculate the relative molecular mass (Mr) of the compound calcium hydroxide, Ca(OH)2

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Problem Solving : Relative Molecular Mass Calculations

The problem: Chris the Chemist has just finished a detailed analysis of a molecule and has found that it is made up of two elements, phosphorus and oxygen.
The molecule contains two phosphorus atoms and has a relative molecular mass of 141.94.

Determine the molecular formula for this molecule.

Solving the Problem

Using the StoPGoPS model for problem solving:

STOP!
State the question. What is the question asking you to do?
Determine the molecular formula for the molecule

What chemical principle will you need to apply?
Apply stoichoimetry (chemical calculations)

What information (data) have you been given?

  • elements in molecule are phosphorus and oxygen
  • 2 atoms of phosphorus
  • Mr(molecule) = relative molecular mass of molecule = 141.94
PAUSE!
Plan. Step 1: Write the symbols for all the elements present in the molecule

Use the Periodic Table to find the symbols for:

phosphorus

oxygen

Step 2: Write a partial chemical formula based on the data provided

Molecule contains only phosphorus and oxygen atoms

2 phosphorus atoms: phosphorus2

let b equal the number of oxygen atoms: oxygenb

Partial chemical formula is phosphorus2oxygenb

substitute in chemical symbols for phosphorus and oxygen

Step 3: Calculate total mass of phosphorus present in molecule

Determine the number of atoms of phosphorus in the molecule = 2

Use Periodic Table to find the relative atomic mass of phosphorus

Mr(phosphorus)

Calculate the total mass of phosphorus atoms present in the molecule:

total mass (phosphorus) = number of phosphorus atoms × relative atomic mass (phosphorus)

total mass (phosphorus) = 2 × Mr(phosphorus)

Step 4: Calculate the total mass of oxygen atoms present in the molecule

mass(molecule) = total mass of phosphorus atoms + total mass of oxygen atoms

Mr(phosphorus2oxygenb) = 2 × Mr(phosphorus) + [b × Mr(oxygen)]

so, total mass of oxygen atoms present = [b × Mr(oxygen)] = Mr(phosporus2oxygenb) - 2 × Mr(phosphorus)

so, total mass of oxygen atoms present = [b × Mr(oxygen)] = 141.94 - 2 × Mr(phosphorus)

Step 5: Calculate the number of oxygen atoms present

Use the Periodic Table to find the relative atomic mass of oxygen

Mr(oxygen)

Substitute this value into the equation for the total of oxygen mass in the molecule:

total mass of oxygen atoms present = b × Mr(oxygen)

therefore, number of oxygen atoms = b = total mass of oxygen ÷ Mr(oxygen)

Step 6: Write the chemical formula for the molecule

Substitute the chemical symbols for phosphorus and oxygen, and, the value for b into the chemical formula for the molecule

phosphorus2oxygenb

GO!
Go with the Plan Step 1: Write the symbols for all the elements present in the molecule

Use the Periodic Table to find the symbols for:

phosphorus: P

oxygen: O

Step 2: Write a partial chemical formula based on the data provided

Molecule contains only phosphorus and oxygen atoms

2 phosphorus atoms: phosphorus2

let b equal the number of oxygen atoms: oxygenb

Partial chemical formula is phosphorus2oxygenb

substitute in chemical symbols for phosphorus and oxygen

P2Ob

Step 3: Calculate total mass of phosphorus present in molecule

Determine the number of atoms of phosphorus in the molecule = 2

Use Periodic Table to find the relative atomic mass of phosphorus

Mr(phosphorus) = 30.97

Calculate the total mass of phosphorus atoms present in the molecule:

total mass (phosphorus) = number of phosphorus atoms × relative atomic mass (phosphorus)

total mass (phosphorus) = 2 × Mr(phosphorus)

total mass (phosphorus) = 2 × 30.97 = 61.94

Step 4: Calculate the total mass of oxygen atoms present in the molecule

mass(molecule) = total mass of phosphorus atoms + total mass of oxygen atoms

Mr(phosphorus2oxygenb) = 2 × Mr(phosphorus) + [b × Mr(oxygen)]

so, total mass of oxygen atoms present = [b × Mr(oxygen)] = Mr(P2Ob) - 2 × 30.97

so, total mass of oxygen atoms present = [b × Mr(oxygen)] = 141.94 - 2 × 30.97

total mass of oxygen atoms present = 141.94 - 61.94 = 80.00

Step 5: Calculate the number of oxygen atoms present

Use the Periodic Table to find the relative atomic mass of oxygen

Mr(oxygen) = 16.00

Substitute this value into the equation for the total of oxygen mass in the molecule:

total mass of oxygen atoms present = b × 16.00 = 80.00

therefore, number of oxygen atoms = b = total mass of oxygen ÷ Mr(oxygen) = 80.00 ÷ 16.00 = 5

Step 6: Write the chemical formula for the molecule

Substitute the chemical symbols for phosphorus and oxygen, and, the value for b into the chemical formula for the molecule

P2Ob

P2O5

PAUSE!
Ponder plausability. Does this solution answer the question that was asked?

Yes, we have determined the molecular formula for this compound.

Is the solution reasonable?

Work backwards by calculating the relative molecular mass for the molecular formula we have found:

Mr(P2O5) = 2 × Mr(P) + 5 × Mr(O) = 2 × 30.97 + 5 × 16.00 = 61.94 + 80.00 = 141.94

Since the relative molecular mass calculated using our chemical formula is the same as the one given in the question, we are satisfied that the answer is correct.

STOP!
State the solution. The molecular formula for this compound is P2O5

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Footnotes:

(1) recommended IUPAC symbol for relative molecular mass (relative molar mass) is Mr.
Debate still rages about whether the term atomic mass, atomic weight, relative atomic mass, relative atomic weight, or standard atomic weight should be used. Don't worry about it, at High School you can treat all these terms as meaning the same thing.
M is the IUPAC symbol recommended for molar mass

(2) The IUPAC periodic table uses standard atomic weights and provides the lower and upper bounds of the standard atomic weight for some elements.
Prior to 1961 the unit amu (atomic mass unit) was used. Since then the unit u (unified atomic mass unit) or Da (dalton) have been widely used but neither of these is an SI unit (as defined by the CIPM, International Committee for Weights and Measures). The issue will be further complicated if the dalton is redefined as being 0.001/NA kg since it would no longer be a unit of atomic mass relative to carbon-12. For High School students it is probably best to avoid the problems and use relative atomic mass (atomic weight) as a quantity with no units.
Molar mass, M, does have units. The SI units for molar mass are kg mol-1