Relative Atomic & Molecular Mass | A-level Chemistry | OCR, AQA, Edexcel

Introduction

In this tutorial, we will explore the concepts of relative atomic and molecular mass, as discussed in the SnapRevise video. Understanding these concepts is essential for A-level Chemistry and is relevant for various exam boards including OCR, AQA, and Edexcel. We will break down complex ideas into manageable steps, highlighting key definitions, calculations, and examples.

Step 1: Understand Relativity in Chemistry

• Atoms are incredibly small, measuring around 10^-8 cm in size.
• Mass is often expressed in relative terms, particularly in relation to the carbon-12 isotope, which serves as the international standard.
• Atomic mass is measured in Unified Atomic Mass Units (u).
  • Carbon-12 (C-12) is defined as 12 u.
  • 1/12th of the mass of C-12 is equivalent to 1 u, which is approximately 1.660540210 x 10^-27 kg.

Step 2: Learn About Relative Isotopic Mass

• Relative isotopic mass refers to the mass of an atom of an isotope compared to 1/12th of the mass of a carbon-12 atom.
• For isotopes, the relative isotopic mass is equal to the mass number.
• Example: Chlorine has two stable isotopes:
  • Chlorine-35 and Chlorine-37.

Step 3: Calculate Relative Atomic Mass

• The relative atomic mass is the weighted mean mass of an atom of an element compared to 1/12th of the mass of a carbon-12 atom.
• Many elements consist of a mixture of isotopes, which must be considered in calculations.
• Example Calculation for Chlorine:
  • Chlorine has isotopic abundances of 75.77% for Cl-35 and 24.23% for Cl-37.
  • Use the formula: [ \text{Relative Atomic Mass} = \frac{(75.77 \times 35) + (24.23 \times 37)}{100} ]
  • Calculation:
    • (2651.95 + 896.51 = 3548.46)
    • Dividing by 100 gives a relative atomic mass of 35.49.

Step 4: Understand Relative Molecular Mass

• The relative molecular mass refers to the mass of a molecule, typically for simple molecular compounds.
• For example, in the molecule (Cl_2):
  • The mass number of chlorine is approximately 35.45.
  • The molecular mass is calculated as: [ \text{Relative Molecular Mass} = 35.45 \times 2 = 70.9 ]

Step 5: Calculate Relative Formula Mass

• The relative formula mass is similar to molecular mass but applies to ionic compounds.
• Example Calculation for Sodium Chloride (NaCl):
  • Sodium (Na) has a relative atomic mass of 23.
  • Chlorine (Cl) has a relative atomic mass of 35.45.
  • The formula mass is calculated as: [ \text{Relative Formula Mass} = 23 + 35.45 = 58.45 ]

Conclusion

In summary, understanding relative atomic and molecular mass involves comparing masses of atoms and molecules to that of carbon-12. Key takeaways include:

• Relative isotopic mass is linked to individual isotopes.
• Relative atomic mass considers isotopic abundance for elements with multiple isotopes.
• Relative molecular mass applies to simple molecules, while relative formula mass is used for ionic compounds.

Next steps could involve practicing calculations with different elements and compounds to reinforce these concepts, as well as reviewing related topics in your A-level Chemistry syllabus.