Understanding Moles in Chemistry

What is a Mole?

• A mole is a unit used to measure the amount of chemical substance.
• Similar to measuring distance in meters or time in seconds.
• Definition: One mole contains $6.02 \times 10^{23}$ particles (atoms, molecules, ions, electrons).
• This number is known as Avogadro's constant.

Relation to Mass and Relative Formula Mass

• The mass of one mole of a substance in grams is numerically equal to its relative atomic or formula mass.
  • Example:
    • Carbon: Relative atomic mass = 12. One mole weighs 12 grams.
    • Oxygen: Relative formula mass = 16 (\times 2 = 32). One mole weighs 32 grams.
    • CO₂: Relative formula mass = 12 + 16 + 16 = 44. One mole weighs 44 grams.

Calculating Moles

• Formula: Number of moles = ( \frac{\text{mass of substance}}{\text{relative atomic/formula mass (Mr)}} )
  • Example:
    • To find moles in 42.5 grams of ammonia (NH₃):
      • Mass = 42.5g, Mr = 14 + (3 \times 1) = 17
      • Moles = ( \frac{42.5}{17} = 2.5 )
    • There are 2.5 moles in 42.5 grams of ammonia.

Calculating Mass from Moles

• Rearrange the formula to find mass if moles are given.
  • Example:
    • Mass of 3 moles of CO₂:
      • Moles = 3, Mr = 44
      • Mass = 3 \times 44 = 132 grams

Mass of an Element in a Compound

• Calculate the mass of a particular element within a compound.
  • Example:
    • Mass of carbon in 3 moles of CO₂:
      • Moles = 3, M (carbon) = 12
      • Mass = 3 \times 12 = 36 grams of carbon.
    • Calculate oxygen mass by subtracting carbon mass from total mass.
      • 132g (total) - 36g (carbon) = 96g of oxygen.

Chemical Equations and Moles

• Chemical equations can be interpreted in terms of moles.
  • Example:
    • ( \text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2 )
    • 1 mole of Mg reacts with 2 moles of HCl to produce 1 mole of MgCl₂ and 1 mole of H₂.
    • Ratios: If 2 moles of Mg, then react with 4 moles of HCl, yielding 2 moles of MgCl₂ and H₂.

Conclusion

• Understanding moles helps in calculating chemical quantities and interpreting chemical equations.
• Use these calculations to predict mass and number of moles in chemical reactions.

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These notes capture the essential information from the lecture on the concept of moles in chemistry and how to apply it in calculations related to mass and chemical reactions. Use them to reinforce your understanding and for reference in solving related problems.