Chemistry

Name: Date: KEYstoich.03a Stoichiometry: Limiting & Excess Reactants

BackgroundUp to now, you’ve been given the amount of one chemical in a single, double, or combustion reaction. However, in the real world, you are often confronted with amounts of two reactants and have to determine which reactant limits the reaction, which reactant is left in excess when the reaction is completed, and how much more of the limiting reactant would you need to use up all of the reactants.

Reference1. Textbook: pp. 296-304.2. The following video: Bozeman Stoichiometry Chemical Essentials 028 (https://www.youtube.com/watch?

v=LQq203gyftA3. The PhET Simulation making sandwiches and chemical reactions.

(http://phet.colorado.edu/sims/html/reactants-products-and-leftovers/latest/reactants-products-and-leftovers_en.html).

ExampleYou are given 10.0 g of methane (CH4) and 10.0 g of oxygen (O2) for the combustion of methane. CH4 + 2O2 CO2 + 2H2O

Solution Format: molar mass A (mole ratio) molar mass B

Grams A x Moles A x Moles B x Moles B = Grams BMoles A

It doesn’t matter which product you choose for your calculations. For CO2:

CH4: 10.0gCH 4

1x

1molCH 4

16.05 gCH 4x

1molCO2

1molCH 4x

44.01 gCO2

1molCO2=27.4 gCO2

O2: 10.0gO2

1x

1molO2

32.00 gO 2x

1molCO2

2molO2x

44.01 gCO2

1molCO2=6.88 gCO2

Limiting Reactant = O2 Excess Reactant = CH4 Moles of CO2 made: 6.88 g

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Chemistry Stoichiometry – Limiting Reactants p. 2

ProblemsIron(III) reacts completely with sulfur in a synthesis reaction. Sulfur (60.0 grams) react with

80.0 grams of iron. How, what is the mass of iron (III) sulfide? Which is the limiting reactant? The excess reactant?

Zinc reacts with sulfuric acid in a single replacement reaction. What is the mass of hydrogen produced from 50.0 g zinc and 50.0 g sulfuric acid?

Chemistry Stoichiometry – Limiting Reactants p. 3

Magnesium (12.4 g Mg) reacts with 38.0 g HCl in a single replacement reaction. What is the mass of the MgCl2 produced? Which is the limiting reactant? Which is the excess?

Barium (13.5 g) reacts with 18.9 g Al(OH)3 in a single replacement reaction. What is the limiting reactant? Which is the excess reactant?

Chemistry Stoichiometry – Limiting Reactants p. 4

Zinc(II) hydroxide (Zn(OH)2; 203 g) reacts with 468 g hydrochloric acid in a double replacement reaction. What is the limiting reactant? What is the excess reactant?

Magnesium (0.0254 g) reacts with 0.25 L oxygen gas in a synthesis reaction. What is the mass of magnesium oxide produced? What is the limiting reactant? What is the excess reactant? (HINT: Use 22.4 L = 1 mol for the oxygen gas instead of its molar mass.)

Chemistry Stoichiometry – Limiting Reactants p. 5

To determine the amount of excess reactant remaining:a. As you’ve done above, determine the amount of product that can be produced from the

limiting reactant.b. From the amount of product that could have been produced from the excess reactant,

subtract the amount of product actually made. This gives you the amount of product that could have been if the limiting reactant had not been depleted.

c. Using stoichiometric calculations, determine the amount of excess reactant that could have been made from the remaining product.

In the above example, where CH4 was the excess reactant:27.4 g CO2 – 6.88 g CO2 actually made) = 20.52 g CO2

20.52gCO2

1x

1molCO2

44.01gCO2x

1molCH4

1molCO2x

16.05gCH 4

1molCH 4

= 7.48 g CH4

Because we started with 10.0 g CH4, 7.48 g CH4 seems reasonable not to have reacted.

Chemistry Stoichiometry – Limiting Reactants p. 6

3Fe(s) + H2O(g) Fe3O4(s) + 4H2(g). You have 67.0 g Fe(s) and 36.0 g H2O(g). determine (a) the limiting reagent, (b) the number of moles of Fe3O4 produced, (c) the number of grams of H2 produced, and (d) the number of grams of excess reagent left over in the reaction

Chemistry Stoichiometry – Limiting Reactants p. 7

Given the following equation:   Al2(SO3)3   +  6 NaOH  ------> 3 Na2SO3    +  2 Al(OH)3

If 10.0 g of Al2(SO3)3 is reacted with 10.0 g of NaOH, determine (a) the limiting reagent, (b) the number of moles of Al(OH)3 produced, (c) the number of grams of Na2SO3 produced, and (d) the number of grams of excess reagent left over in the reaction.