Mole Concept and Stoichiometry — CDS Chemistry

Formulas (3)

• Moles, molar mass and Avogadro's number · Moles and particles
  $$n = \frac{m}{M} \qquad N = n \times N_A, \quad N_A = 6.022 \times 10^{23}$$
• Equivalent weight · Equivalent weight
  $$E = \frac{M}{n\text{-factor}}$$
• Molarity and molality · Molarity and molality
  $$M = \frac{n_{\text{solute}}}{V_{\text{solution}}\,(\text{L})} \qquad m = \frac{n_{\text{solute}}}{m_{\text{solvent}}\,(\text{kg})}$$

Watch out for (8)

• Count atoms, not molecules, when asked for atoms
  → Moles, molar mass and Avogadro's number
• Equal in number is not equal in mass
  → Moles, molar mass and Avogadro's number
• Helium is monatomic
  → Moles, molar mass and Avogadro's number
• A mole is a number, not a mass
  → Moles, molar mass and Avogadro's number
• Divide by the n-factor, not by the number of atoms
  → Equivalent weight
• Basicity counts only replaceable hydrogens
  → Equivalent weight
• Molarity uses the solution, molality the solvent
  → Molarity and molality
• Molality does not change with temperature
  → Molarity and molality

Formulas (2)

• Balancing a chemical equation · Conservation of atoms
  $$\sum \text{atoms of each element (reactants)} = \sum \text{atoms of that element (products)}$$
• Mole ratios and the limiting reactant · Mole ratio from coefficients
  $$aA + bB \rightarrow cC: \qquad n_C = \frac{c}{a}\, n_A$$

Watch out for (5)

• Re-check the first element after fixing the second
  → Balancing a chemical equation
• Never change a subscript
  → Balancing a chemical equation
• The reactant in excess does not set the yield
  → Mole ratios and the limiting reactant
• Mass is conserved, moles need not be
  → Mole ratios and the limiting reactant
• Per gram means divide by mass
  → Mole ratios and the limiting reactant