🧪 GCSE Chemistry Revision Newsletter - Relative Formula Mass

Part 1 - Keywords:

• Relative Formula Mass

• Relative Atomic Mass

• Compound

• Chemical Equation

• Percentage by Mass

Part 2 - Key Facts:

What is Relative Formula Mass (Mr)?

• Relative formula mass (Mr) of a compound is the sum of the relative atomic masses of all the atoms in the compound.

• The relative atomic mass (Ar) is the weighted average mass of an atom of an element compared to 1/12 of the mass of a carbon-12 atom.

• For example, the relative formula mass of water (H₂O) is:

  • H (Hydrogen) = 1

  • O (Oxygen) = 16

  Mr=(2×1)+(1×16)=18 So, the relative formula mass of water (H₂O) is 18.

Balanced Chemical Equations:

• In a balanced chemical equation, the sum of the relative formula masses of the reactants must equal the sum of the relative formula masses of the products.

• This is a direct consequence of the law of conservation of mass, which states that no atoms are lost or gained during a reaction.

• For example, if the reaction is:
  2H₂+O2→2H₂O
  The total relative formula mass of the reactants is: (2×2)+(1×16)=4+16=20 And the total relative formula mass of the products is: (2×18)=36  So, the mass is conserved in this reaction.

Part 3 - Example Calculation:

Example 1: Finding Mr of a Compound

Let’s calculate the relative formula mass of sodium carbonate (Na₂CO₃):

• Sodium (Na) = 23

• Carbon (C) = 12

• Oxygen (O) = 16

Mr=(2×23)+(1×12)+(3×16)=46+12+48=106 MrMr = (2 \times 23) + (1 \times 12) + (3 \times 16) = 46 + 12 + 48 = 106 \, \text{Mr}

So, the relative formula mass of sodium carbonate is 106.

Example 2: Percentage by Mass Calculation

Let’s calculate the percentage by mass of carbon in sodium carbonate (Na₂CO₃):

• Relative formula mass of Na₂CO₃ = 106

• The mass of carbon in Na₂CO₃ is 12 (from 1 carbon atom).

Percentage by mass of carbon=(12106)×100=11.32%\text{Percentage by mass of carbon} = \left( \frac{12}{106} \right) \times 100 = 11.32\%

So, the percentage by mass of carbon in sodium carbonate is 11.32%.

Part 4 - Quick Quiz:

1. What is the relative formula mass of H₂O?

   • A) 16

   • B) 18

   • C) 20

2. The relative atomic mass of sodium (Na) is 23. What is the relative formula mass of NaCl?

   • A) 35

   • B) 58

   • C) 45

3. What does the law of conservation of mass tell us about balanced chemical equations?

   • A) The mass of the reactants equals the mass of the products.

   • B) The mass of the reactants is greater than the mass of the products.

   • C) The mass of the products is greater than the mass of the reactants.

Answers:

1. B

2. B

3. A

Part 5 - Challenge Yourself:

Task:
Calculate the percentage by mass of oxygen in carbon dioxide (CO₂). The relative atomic mass of carbon is 12, and oxygen is 16.

Part 6 - Revision Tips:

• Always check the subscript numbers in the formula to determine how many atoms of each element are present.

• Relative formula mass (Mr) is just the sum of the relative atomic masses of all atoms in the formula.

• Percentage by mass is a helpful way to analyse the composition of compounds.

Part 7 - Real-World Connection:

Relative formula mass and percentage by mass are used in industries like pharmaceuticals to calculate the proportions of elements in drugs and materials science to understand the composition of materials.

Part 8 - More Help:

Here are two useful webpages and one YouTube video to help you revise the GCSE Chemistry topic of Relative Formula Mass:

1. Save My Exams - Relative Formula Mass
   Save My Exams
   This page provides detailed notes on calculating relative formula mass, including worked examples and practice questions to reinforce your understanding.

2. MME Revise - Relative Formula Mass
   MME Revise
   Offers clear explanations and step-by-step guides on calculating relative formula mass, along with example questions to test your knowledge.

3. YouTube Video - GCSE Chemistry - Relative Formula Mass
   YouTube Video
   This video covers how to find the relative formula mass, which is the sum of the relative atomic masses of all the atoms within a molecule, with practical examples to aid understanding.

These resources should assist you in mastering the concept of relative formula mass in chemistry.

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Until next time, keep working on your GCSE Science skills!

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