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 ! ! !

Example 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?

 

Example 2: 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?

Example 3:

Liquid bromine is reacted with aluminum to produce aluminum bromide.

2Al(s) + 3Br2 (l) 2AlBr3 (s)

If 450 ml of bromine reacts with excess aluminum, how much aluminum bromide will be made? The density of bromine is 3.1 g/ml

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.

Example 4:

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?

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.)

Example 5:

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?

Solution Stoichiometry

Solutions

Uniform mixtures, composition is the same throughout

Solvent—substance doing the dissolving, present in a larger amount.

Solute—dissolved substance, present in a smaller amount

Examples: sugar in tea, salt water

Concentration Amount of solute

amount of solvent or solution

How much “stuff” is in a solution

Solute quantity in a certain amount of solvent/solution

Usually seen as mass percent value

Ex. Orange juice, sweet tea

Molarity (M)

Main unit of concentration in chemistry

Moles solute

L solution

Many solutions provide only the % solute by mass need to know density to get molarity

Example 6:

What is the concentration/molarity of a 50.0 ml solution with 1.25g of CuSO4 in water?

Example 7:

Luke Skywalker has a stock solution of aqueous formic acid that is 90% HCOOH by mass. The density of this solution is 1.20 g/ml. Skywalker wants to know the molarity of this solution to determine if it can be used in the construction of his latest project: starship destroyer X.

More Molarity Practice

1) A 500 ml solution of NaOH has a molarity of 0.50M. How many moles of NaOH are in the solution?

2) You have two solutions. One solution has 500 ml of 0.20 M NaCl and the other solution has 250 ml of 0.25 M NaCl.Which solution is more concentrated?

3) 2.75g of AgNO3 dissolved in 0.250 L of solution. Find the molarity.

4) How many grams of H2SO4 are needed to make a 5M concentration solution with 250 ml?

Dilutions

Often want to prepare solutions of LESS concentration than stock solution

Technique of preparing a less concentrated solution by adding more solvent to decrease the solution’s concentration

Solute amount remains the same SOOO

M1V1 = M2V2

M1/V1 values indicate initial “concentrated” solution

M2/V2 values indicate final “dilute” solution

Example 8:

Baking soda (NaHCO3) neutralizes HCl in the following reaction when 175 ml of 1.55 M NaHCO3 is added to 235 ml of 1.22 M HCl.

NaHCO3 (aq) + HCl (aq) NaCl (aq) + H2O (l) + CO2 (g)

a) How many grams of CO2 are given off?

b) What is the molarity of the aqueous NaCl solution?

Example 9:

Household bleach can be produced based on the chemical reaction below.

2NaOH(aq) + Cl2 (g) NaOCl (aq) + NaCl (aq) + H2O (l)

a) What volume of 0.25M NaOH is required to produce 18g of NaOCl?

b) what volume of 0.25M NaOH is required to react with 0.32 moles of chlorine?

c) If 4.00L of 0.25M NaOH are used with excess chlorine gas, how much NaOCl is produced?

Homework:

Finish Stoichiometry Sheet

Percent Water Lab Report