Stoichiometry Overview

Introduction

• Revisited stoichiometry in the context of concentration and solutions.
• Central concept: Moles and the Mole Ratio.

Stoichiometric Map

• Various pathways to moles:
  • Mass
  • Volume of gas at standard temperature and pressure (STP)
  • Representative particles
• Molarity: Molarity (M) = Moles of solute / Liters of solution
  • If concentration and volume of a solution are known, moles of solute can be calculated.

Example Problem: Ammonium Nitrate Production

• Question: How many grams of ammonium nitrate can be produced from:
  • 55.0 mL of 3.2 M ammonium carbonate solution
  • 100 mL of excess aqueous copper(II) nitrate?

Steps to Solve:

1. Balanced Chemical Equation:
   • Ammonium carbonate + Copper(II) nitrate → Ammonium nitrate + Copper carbonate (precipitate)
2. Identify Known and Unknown:
   • Known: 55 mL of 3.2 M ammonium carbonate
   • Unknown: grams of ammonium nitrate
3. Molar Mass Calculation: Calculate molar mass of ammonium nitrate (80.06 g/mol).
4. Conversion Process:
   • Volume of solution (mL) to liters: 55 mL = 0.055 L
   • Use concentration to find moles of ammonium carbonate:
     • Moles = Molarity × Volume = 3.2 mol/L × 0.055 L = 0.176 moles of ammonium carbonate.
   • Use mole ratio from balanced equation (1:1 for ammonium carbonate to ammonium nitrate).
   • Moles of ammonium nitrate = 0.176 moles.
5. Convert Moles to Grams:
   • Grams of ammonium nitrate = Moles × Molar Mass = 0.176 moles × 80.06 g/mol = 14.09 grams.

Final Volume Calculation:

• Total volume after mixing = 55 mL + 100 mL = 155 mL = 0.155 L.
• Calculate concentration of ammonium nitrate in final solution:
  • Moles of ammonium nitrate = 0.176 moles.
  • Concentration = Moles / Volume = 0.176 moles / 0.155 L = 1.13 M.

Bonus Question

• Concentration of ammonium nitrate in the final solution was calculated to be approximately 1.13 M.

Limiting Reactant Problem

• Task: Identify limiting reactant in a reaction between silver nitrate and magnesium chloride.

Steps to Solve:

1. Balanced Chemical Equation:
   • 2 AgNO₃ + MgCl₂ → 2 AgCl (precipitate) + Mg(NO₃)₂
2. Given Information:
   • Silver nitrate concentration = 0.150 moles
   • Magnesium chloride concentration = 0.0875 moles
3. Convert to Moles:
   • Use molarity equation for both reactants.
4. Stoichiometric Conversion:
   • Using the mole ratio (1:1) to find moles of magnesium chloride needed for 0.150 moles of silver nitrate:
     • 0.150 moles AgNO₃ × (1 mole MgCl₂ / 2 moles AgNO₃) = 0.075 moles of MgCl₂ needed.
5. Determine Limiting Reactant:
   • Compare available moles of magnesium chloride (0.0875 moles) with moles needed (0.075 moles).
   • Conclusion: Silver nitrate is the limiting reactant.

Conclusion

• This lecture covered stoichiometry in solution chemistry, focusing on calculating moles, grams, and identifying limiting reactants in chemical reactions.