Stoichiometry

The Basics What is stoichiometry?

A method of determining the quantities of products produced in a chemical reaction or what amount of a reactant needed to carry out a chemical reaction.

What do the stoichiometric coefficients tell us about the chemical reaction and the ratio between the chemical compounds involved?

MOLE RATIO ! ! !

Ex. 1 Milk of magnesia, Mg(OH)2, reacts with stomach acid in a neutralization reaction. What is the mass (in grams) of magnesium chloride produced when 3.00g of milk of magnesia is taken?

More Practice with the Basics

1) _____N2H4 + ____O2 ____N2 + _____H2O (N2H4 is rocket fuel)

We have 5.20 mol of N2H4. How many moles of O2 do we use up in the reaction?

2) ____C6H12O6 + ____O2 ____CO2 + _____H2O

If you eat 10.0 grams of sugar, how many grams of oxygen do you need to breathe to carry out this reaction and get energy?

 

True Test of Stoichiometry Skills

Gaseous ammonia is placed into an aqueous solution that is 65% H2SO4 by mass with a density of 1.55 g/mol. Ammonium sulfate is produced from this reaction and used as fertilizer. How many milliliters of sulfuric acid are needed along with 1.00 kg NH3 to produce ammonium phosphate?

Limiting Reactant Chemical compound that “limits” or determines

how much product can be produced

All of the compound is consumed in the reaction

Excess reactant—chemical compound not consumed in the reaction, plenty of the compound is available for the chemical reaction

Steps to find the limiting reactant

Two stoichiometry problems in ONE !

1) Write a balanced equation

2) Find the amount of product (in grams) produced from each reactant amount.

3) The reactant producing the SMALLEST product is the limiting reactant.

Ex. 2 Aspirin synthesis:

2C7H6O3 + C4H6O3 2C9H8O4 + H2O

When 20.0g of C7H6O3 and 20.0g of C4H6O3 react, which is the limiting reactant? What is the mass (in grams) of aspirin produced?

Limiting Reactant—More Practice

1) For the thermite reaction between chromium(III) oxide and aluminum metal, producing chromium metal and aluminum oxide, what is the maximum weight of metallic chromium that can be produced by reacting 38.0 grams of Cr2O3 with 9.00 grams of metallic aluminum?

2) What mass of of magnesium oxide is formed when 10.0g of magnesium metal reacts with 10.0g of oxygen?

Percent Yield “yield—” the amount of product actually made

through a chemical reaction. Why is this value important?

Theoretical yield— calculated amount of product

Actual/Experimental yield– the “true/actual” amount of product made in the chemical reaction, usually less than theoretical

How is this value different than percent error?

Percent Yield

= Actual yield x 100%

Theoretical yield

Ex. 98.80% of Mg3N2 was produced in a chemical reaction. What happened to the rest? (purity, other reactions, remaining product, etc.)

Consider the reaction between solutions of barium hydroxide and phosphoric acid, which forms water and quantitatively precipitates

barium phosphate. A student mixes 500.0 mL of 0.0400 M H3PO4 with 240.0 mL of 0.100 M Ba(OH)2 and collects 4.66 grams of a white

precipitate.

 1) Which reactant was the limiting reagent?

2) What was the theoretical amount of precipitate that should have been formed?

3) What is the percent yield for this experiment?

Homework Read over lab procedure

Read pp. 97-106

Stoichiometry Worksheet