Stoichiometry Notes

Proportional Relationships

• Analogy: Cheese Sandwich
  • Recipe: 2 Bread (Bd) + 2 Cheese (Ch) + 1 Lettuce (L) → 1 Sandwich (Bd2Ch2L)
  • Question: If you have 4 Bd, 4 Ch, and 2 L, how many sandwiches can you make?

Stoichiometry

• Definition: The ratios between coefficients in a chemical reaction, based on the mole ratio.
• Example: $2 Mg + O_2 \rightarrow 2 MgO$
• Stoichiometric coefficients are used to determine mole ratios.

Stoichiometry Problems

• Problem 1: How many moles of $KClO_3$ must decompose to produce 9 moles of oxygen gas?
• Reaction: $2KClO<em>3 \rightarrow 2KCl + 3O</em>2$
• Given: 9 moles of $O_2$
• Calculation:
  $9 \, mol \, O<em>2 \times \frac{2 \, mol \, KClO</em>3}{3 \, mol \, O<em>2} = 6 \, mol \, KClO</em>3$
• Start with the given number, use a ratio to cancel out unwanted units.

Stoichiometry Problems Continued

• Reaction: $CaCl<em>2 \rightarrow Ca + Cl</em>2$
  • Problem: How many moles of calcium metal are produced from the decomposition of 8 moles of calcium chloride?
  • Calculation:
    $8 \, mol \, CaCl<em>2 \times \frac{1 \, mol \, Ca}{1 \, mol \, CaCl</em>2} = 8 \, mol \, Ca$
  • Problem: How many moles of calcium metal and chlorine gas are produced from the decomposition of 8 moles of calcium chloride?
  • Calculation:
    $8 \, mol \, CaCl<em>2 \times \frac{1 \, mol \, Cl</em>2}{1 \, mol \, CaCl<em>2} = 8 \, mol \, Cl</em>2$
  • Result: 8 moles of calcium metal and 8 moles of chlorine gas are produced.

More Stoichiometry Problems

• Reaction: $2 Sb + 3 Cl<em>2 \rightarrow 2 SbCl</em>3$
  • Problem: How many moles of chlorine gas are required to react with 5 moles of antimony?
  • Calculation:
    $5 \, mol \, Sb \times \frac{3 \, mol \, Cl<em>2}{2 \, mol \, Sb} = 7.5 \, mol \, Cl</em>2$
  • Problem: How many moles of $SbCl<em>3$ are produced from 5 moles of antimony and excess $Cl</em>2$?
  • Calculation:
    $5 \, mol \, Sb \times \frac{2 \, mol \, SbCl<em>3}{2 \, mol \, Sb} = 5 \, mol \, SbCl</em>3$
  • Problem: How many moles of $SbCl<em>3$ are produced from 7.5 moles of $Cl</em>2$ and excess Sb?
  • Calculation:
    $7.5 \, mol \, Cl<em>2 \times \frac{2 \, mol \, SbCl</em>3}{3 \, mol \, Cl<em>2} = 5 \, mol \, SbCl</em>3$

Stoichiometry with Larger Coefficients

• Reaction: $2C<em>6H</em>6 + 15O<em>2 \rightarrow 12 CO</em>2 + 6H_2O$
  • Problem: How many moles of oxygen gas are required to react with 5.00 moles of $C<em>6H</em>6$?
  • Calculation:
    $5 \, mol \, C<em>6H</em>6 \times \frac{15 \, mol \, O<em>2}{2 \, mol \, C</em>6H<em>6} = 37.5 \, mol \, O</em>2$
  • Problem: How many moles of $CO<em>2$ are produced from 5 moles of $O</em>2$ and excess $C<em>6H</em>6$?
  • Calculation:
    $5 \, mol \, O<em>2 \times \frac{12 \, mol \, CO</em>2}{15 \, mol \, O<em>2} = 4 \, mol \, CO</em>2$