Limiting Reactant Answers

Chemistry                                                                                                                         Guided Instructional Activity 6-6
Unit 6: Chemical Reactions and Stoichiometry                                                                                                    Chapter 12
Working Mode: Pairs                                    

Learning objectives: To use the stoichiometric coefficients of a balanced chemical equation along with molar masses to solve limiting reactant stoichiometry problems.  To visualize the changes that occur as the reaction proceeds to completion.

Background: Limiting reactant stoichiometry problems are characterized by the fact that quantities of all reactants are initially present.  The amounts of reactants are not present in stoichiometric quantities.  The amount of products formed is based on the reactant that is used up first, which is identified as the limiting reactant.  Once the limiting reactant is used up, no more products will be formed.  The other reactant is identified as the excess reactant.   The unused excess reactant remains in the reaction mixture.


Activity: The balanced equation below shows the reaction between solid and calcium water to form aqueous calcium hydroxide and hydrogen gas.

                                                Ca(s)+ 2 H2O(l) ® Ca(OH)2(aq) +  H2(g)

A reaction starts with 150.0 grams of calcium and 150.0 grams of water.  Use the balanced equation and the molar masses, below, to carry out the following tasks. Show all work in detail.

 

Molar Masses

H2O     18.02 g/mol

Ca(OH)2  74.10 g/mol

Ca     40.10 g/mol

   H2  2.016 g/mol

Student 1: Find the moles of calcium present at the start of the reaction.  (Answer: 3.741 mol)

Student 2: Find the moles of water present at the start of the reaction.  (Answer: 8.324 mol)

Student 1: Use the moles of calcium initially present to calculate the number of moles of water required to completely react with the calcium.  (Answer: 7.482 mol H2O required to react with the 150.0 g Ca present.)

Student 2: Use the moles of water initially present to calculate the number of moles of calcium required to completely react with the water.  (Answer: 4.162 mol Ca required to react with the 150.0 g water.)

Student 1: Identify the reactant (limiting reactant) completely consumed in the reaction.  (Ca is limiting.)

Student 2: Find the moles and mass (g) of the excess reactant consumed in the reaction.  (Answer: 7.482 mol of H2O and 134.8 g react.)

Student 1: Find the moles and mass of calcium hydroxide formed.  (Answer: 3.741 mol and 277.2 g Ca(OH)2 formed.)

Student 2: Find the moles and mass of hydrogen formed.  (Answer: 3.741 mol and 7.542 g of H2 formed.)

Student 1: Find the moles and mass of excess reactant remaining in the reaction mixture.  (Answer: 0.842 mol and 15.2 g H2O remain.)

Student 2: Find the moles and mass of the limiting reactant remaining in the reaction mixture.  (0 mol and 0 g of Ca remain.)

Student 1: Draw a diagram of the initial reaction mixture in beaker A.  (Not shown)

Student 2: Draw a diagram of the final reaction mixture beaker B.  (Not Shown)


        Beaker A                                                          Beaker B

 

Based on your solutions to the problem, fill in the following table. 

 

Calcium

Water

Calcium hydroxide

Hydrogen

Initial Mass (g)

150.00

150.00

0

0

Initial mol

 3.741 mol

 8.324 mol

 0

 0

Final mol

 0 mol

 0.842 mol

 3.741 mol

 3.741 mol

Final mass (g)

 0 g

 15.2 g

 277.2 g

 7.542 g

mass(g) reacted/formed

 150.0 g

 134.8 g

 277.2 g

 7.542 g

 

Total Mass (g) reactants reacted

Total Mass (g) products formed

 287.8 g

284.7 g

 

Total Mass (g) of starting materials

Total Mass (g) products and un-reacted starting materials

 300.0 g

 299.9 g