Chemistry Notes on Mole, Mass, and Composition

The Mole

9.1 Relative Masses

• Relative Masses: Commonly used masses compared to a standard, often the isotope Carbon-12 ($^{12}C$).

Relative Molecular Mass

• Elements as Molecules: Examples include $O2$ (oxygen) and $CO2$ (carbon dioxide).
• Relative Molecular Mass ($M_r$): Determined for these molecular substances.

Example: Calculating Relative Molecular Mass of Nitric Acid ($HNO_3$)

• Identify atomic masses from the periodic table:

  • A$_{(H)} = 1.008$
  • A$_{(N)} = 14.01$
  • A$_{(O)} = 16.00$

• Calculate:
  Mr = 1 imes A{(H)} + 1 imes A{(N)} + 3 imes A{(O)}
  M_r = 1.008 + 14.01 + 3 imes 16.00 = 63.02

Relative Formula Mass

• Ionic Compounds: For compounds like $NaCl$, use the term Relative Formula Mass instead of Molecular Mass.

Example: Calculating Relative Formula Mass of Copper(II) Nitrate ($Cu(NO3)2$)

• Atomic masses:

  • A$_{(Cu)} = 63.55$
  • A$_{(N)} = 14.01$
  • A$_{(O)} = 16.00$

• Calculation:
  M{formula} = 1 imes A{(Cu)} + 2 imes A{(N)} + 6 imes A{(O)}
  = 63.55 + 2(14.01) + 6(16.00) = 187.6

9.2 Introducing the Mole

• Mole (mol): Standard unit for the amount of substance indicated by $n$.
  • Example: For hydrogen atoms, $n(H)$.
• Avogadro's Number ($N_A$): 1 mol of substance contains $6.022 imes 10^{23}$ particles.

Examples of Using the Mole to Measure Particles

• $1 mol$ of $H_2$: $2$ moles of $H$ atoms.
• $2 mol$ of $H_2O$ (water):
  • Equivalent to $4$ mol of $H$ atoms.
• $10 mol$ of $C6H{12}O_6$ (glucose): Regular conversions between moles of substances and atoms/ions.

Calculating Moles from Molecules

• Example: To find H atoms from sulfuric acid ($H2SO4$).
  • Given $3.6$ mol, thus:
    n(H) = n(H2SO4) imes 2
  • Result: $n(H) = 3.6 imes 2 = 7.2$ mol.

Relationship Between Avogadro's Number, Particles and Amount

• Formula: n = rac{N}{N_A}
  • Where $N$ is particles, $n$ is moles, and $N_A$ is Avogadro’s number.

Example: Calculate Number of Molecules in 3.5 mol of $H_2O$

• Given:
  • $n(H_2O) = 3.5$ mol
• Use: N = n imes N_A
  • $N(H_2O) = 3.5 imes 6.022 imes 10^{23} = 2.1 imes 10^{24}$ molecules.

9.3 Molar Mass

• Definition: Mass in grams of one mole of substance, denoted by $M$ (g/mol).
  • Molar mass of elements equals their atomic mass in grams.
• Molar Mass for Compounds: Equivalent to relative molecular/formula mass in grams.

Example of Molar Mass Calculation

Calculating Mass of a Substance (Example: $Mg(NO3)2$)

• Given 0.35 mol:
  • Use:
    M(Mg(NO3)2) = 24.31 + (2 imes 14.01) + (6 imes 16.00) = 148.33 g/mol
  • m = n imes M = 0.35 imes 148.33
    ightarrow m = 52 g

9.4 Percentage Composition

• Definition: Proportion by mass of different elements in a compound.

Example: Calculate % Composition of Aluminium in $Al2O3$

• Molar mass: 101.96 g/mol.
• Total mass of Aluminium = $53.96$ g (from $2 imes M(Al)$).
• ext{% of Al} = rac{53.96}{101.96} imes 100 = 52.92 ext{%}

Practice Questions

• Complete assigned chapter review questions for reinforcement.