ChemistryStoichiometry – Limiting Reactantsp. 1

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Stoichiometry: Limiting & Excess Reactants

Background

Up 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.

Reference

1. Textbook: pp. 296-304.

2. The following video: / Bozeman Stoichiometry Chemical Essentials 028 (

3. The PhET Simulation making sandwiches and chemical reactions.

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Example

You 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 B
Moles A

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

CH4:

O2:

Limiting Reactant = O2Excess Reactant = CH4Moles of CO2 made: 6.88 g

Problems

  1. Iron(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?

  1. 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?

  1. 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?

  1. 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?

  1. 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?

  1. 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.)

To determinethe 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
/ = 7.48 g CH4
Because we started with 10.0 g CH4, 7.48 g CH4 seems reasonable not to have reacted.
  1. 3Fe(s) + H2O(g) Fe3O4(s) + 4H2(g). You have 67.0 g Fe(s) and 36.0 g H2O(g). determine (a) thelimiting 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 inthe reaction

  1. 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) thelimiting 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 inthe reaction.