A Level Chemistry Revision "Relative Atomic Mass"

Introduction

This tutorial focuses on understanding relative atomic mass, a fundamental concept in A Level Chemistry. It will guide you through the significance of relative masses, how they relate to carbon-12, and the calculation of relative atomic mass. This knowledge is essential for students preparing for A Level Chemistry exams.

Step 1: Understanding Relative Mass

• Masses in chemistry are always compared to a standard reference, which is carbon-12.
• Carbon-12 is chosen because it is a stable isotope and is abundant in nature.
• The relative atomic mass of an element is the average mass of its isotopes, taking their abundance into account.

Step 2: Exploring Relative Isotopic Mass

• Relative isotopic mass refers to the mass of a specific isotope relative to carbon-12.
• Key features of relative isotopic mass include:
  • It is a dimensionless quantity.
  • It reflects the mass of one atom of the isotope on a scale where carbon-12 is exactly 12.
  • Each isotope of an element will have a different relative isotopic mass.

Step 3: Calculating Relative Atomic Mass

• Relative atomic mass is a weighted average of the relative isotopic masses of an element's isotopes.
• To calculate relative atomic mass, follow these steps:
  1. Identify the isotopes of the element and their relative isotopic masses.
  2. Determine the natural abundance of each isotope (usually given in percentage).
  3. Use the formula:

     Relative Atomic Mass = (isotopic mass1 * abundance1 + isotopic mass2 * abundance2 + ...) / total abundance
     

  4. Example Calculation:
     • For an element with two isotopes:
       • Isotope A: mass = 10, abundance = 75%
       • Isotope B: mass = 11, abundance = 25%
     • Calculation:

       Relative Atomic Mass = (10 * 0.75 + 11 * 0.25) = 10.25
       

Step 4: Practice Examples

• To reinforce your understanding, try these practice problems:
  1. An element has two isotopes:
     • Isotope X: mass = 14, abundance = 60%
     • Isotope Y: mass = 15, abundance = 40%
     • Calculate the relative atomic mass.
  2. A third isotope of the same element has a mass of 16 and an abundance of 10%. How does this affect the overall relative atomic mass?

Conclusion

Understanding relative atomic mass and related concepts such as relative isotopic mass is crucial for A Level Chemistry. By mastering these calculations and their significance, you're better prepared for exams and practical applications in chemistry. Practice the calculations provided and explore further examples to solidify your understanding.