AP Chemistry Questions on Limiting Reactants and Mole Ratios

🧪 Quick Study Guide

• ⚖️ Stoichiometry: The study of the quantitative relationships or ratios between two or more substances undergoing a physical change or chemical change.
• ⚗️ Mole Ratio: The ratio between the amounts in moles of any two compounds involved in a chemical reaction. Mole ratios are derived from the coefficients in a balanced chemical equation.
• 🎯 Limiting Reactant: The reactant that is completely consumed in a chemical reaction and limits the amount of product formed.
• 📈 Excess Reactant: The reactant that is present in a greater amount than necessary to react completely with the limiting reactant.
• 📝 Steps to Identify Limiting Reactant:
  1. Balance the chemical equation.
  2. Convert the given masses of reactants to moles.
  3. Calculate the mole ratio of the reactants.
  4. Determine which reactant is the limiting reactant based on the mole ratio.
• 🧮 Formula Reminders:
  • Moles = $\frac{Mass}{Molar\,Mass}$
  • Mole Ratio: $\frac{Moles\,of\,A}{Moles\,of\,B}$ (from balanced equation)

Practice Quiz

1. What is the mole ratio of $O_2$ to $H_2O$ in the following balanced equation: $2H_2 + O_2 \rightarrow 2H_2O$?
   1. 1:1
   2. 2:1
   3. 1:2
   4. 2:2
2. If 10.0 g of $H_2$ reacts with 32.0 g of $O_2$, which is the limiting reactant in the formation of water ($H_2O$)?
   1. $H_2$
   2. $O_2$
   3. $H_2O$
   4. Neither, they react completely
3. For the reaction $N_2 + 3H_2 \rightarrow 2NH_3$, if you start with 6 moles of $H_2$, how many moles of $N_2$ are required for complete reaction?
   1. 1 mole
   2. 2 moles
   3. 3 moles
   4. 6 moles
4. In the reaction $2CO + O_2 \rightarrow 2CO_2$, if 56 g of CO reacts with 32 g of $O_2$, what is the limiting reactant? (Molar mass of CO = 28 g/mol, Molar mass of $O_2$ = 32 g/mol)
   1. CO
   2. $O_2$
   3. $CO_2$
   4. Neither, they react completely
5. What does the limiting reactant determine in a chemical reaction?
   1. The rate of the reaction
   2. The amount of excess reactant
   3. The maximum amount of product formed
   4. The equilibrium constant
6. For the reaction $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$, if you have 1 mole of $CH_4$ and 3 moles of $O_2$, which reactant is in excess?
   1. $CH_4$
   2. $O_2$
   3. $CO_2$
   4. $H_2O$
7. If 4.0 g of $H_2$ reacts with an excess of $N_2$ to produce ammonia ($NH_3$), what is the theoretical yield of $NH_3$ in grams? ($N_2 + 3H_2 \rightarrow 2NH_3$, Molar mass of $NH_3$ = 17 g/mol)
   1. 22.7 g
   2. 45.3 g
   3. 11.3 g
   4. 5.66 g