Catalyst 1. C3H8 is burned in air 2. A copper (II) wire is submerged in a solution of silver nitrate. 3. Magnesium carbonate is heated

End  

Lecture 3.6 – Stoichiometry, Empirical Formulas, and Limiting Reactants

Today’s Learning Targets   LT 3.10 – When given the starting amount of grams of particular

substance in a chemical reaction, I can convert this to moles, grams, particles, or liters gas of another compound in the same chemical reaction.

  LT 3.11 – I can calculate the empirical formula for a given compound using experimental data. Furthermore, I can discuss what these results indicate.

  LT 3.12 – I can identify the limiting reactant for a reaction. Additionally, I can support my claim through stoichiometric calculations.

Stoichiometry Review   Recall, we can convert grams of one substance to grams of

another substance using stoichiometry.   Label the value that you need to use above each of the arrows

Grams  of  Substance  

A    

Grams  of  Substance  

B    

Moles  of  A  

Moles  of  B

Stoichiometry Review   We also can convert from particles of one substance to particles

of another substance.   Label the conversion factors above the arrows

Atoms  of  Substance  A  

Moles  of  A   Moles  of  B

Atoms  of  Substance  B  

Converting Moles to Liters of Gas   Finally, we can convert from liters of one gaseous substance to

liters of another gaseous substance.   We know that 1 L of any gas occupies 22.4 L

Volume  of  Substance  A  

Volume  of  Substance  B

Moles  of  A  

Moles  of  B

Class Example   I run the following reaction:

N2 (g) +3 H2 (g) 2NH3 (g) If I start with 14 L of N2, then how many L of NH3 should I

produce?

Table Talk   If I run the following reaction:

C6H12 (l) + O2 (g) H2C6H8O4 (l) + H2O (g) I produce 54.8 L of H2O, then how many L of O2 did I begin with?

Grams  of  Substance  A    

Grams  of  Substance  B    

Volume  of  Substance  B

Atoms  of  Substance  

 A  

Moles  of  A  

Moles  of  B

Atoms  of  Substance    

B  

Volume  of  Substance  A  

Grams  of  Substance  A    

Grams  of  Substance  B    

Volume  of  Substance  B

Atoms  of  Substance  

 A  

Moles  of  A  

Moles  of  B

Atoms  of  Substance    

B  

Volume  of  Substance  A  

Fritz Habor Research Assistant

Empirical Formula   The empirical formula is the smallest whole number ratio of

atoms in a compound.   The molecular formula is the actual ratio of atoms in a

compound

C6H12O6 CH2O Molecular Formula

Empirical Formula

Class Example   Ascorbic Acid (vitamin C) contains 40.92% C, 4.58% H, and

54.50% O by mass. What is the empirical formula?   If the molecular mass of ascorbic acid is determined to be 176.12

g/mol, then what is the molecular formula of vitamin C?

Table Talk   A perfume that you are wearing is 70.57% carbon, 5.93%

hydrogen, and 23.50% oxygen. What is the empirical formula of this compound?

  If the molar mass is 136.15 g/mol, then what is the molecular formula?

Combustion Analysis   Combustion problems can also be analyzed using the empirical

formula process Hydrocarbon + O2 CO2 + H2O

  All the carbon from the hydrocarbon is converted into CO2.   All the hydrogen from the hydrocarbon is converted into water   Therefore, the remaining mass in the two molecules represents

the amount of oxygen in the original hydrocarbon

Class Example   Isopropyl alcohol is composed of C, H, and O. Compusition of

0.255 g of isopropyl alcohol produces 0.561 g of CO2 and 0.306 g of H2O. Determine the empirical formula of isopropyl alcohol

Table Talk   Caproic acid is composed of C, H, and O. Combustion of a 0.225

g sample of this compound produces 0.512 g CO2 and 0.209 g H2O. What is the empirical formula?

  The molar mass is 116 g/mol. What is the molecular formula?

Making Bikes   You have 5 bike frames, 12 bike wheels, and 4 handle bars. How

many bikes can you make?   What stops you from making more bikes?

Limiting Reactants   Oftentimes, we do not have equal amounts of reactants,

therefore the reaction will stop when one of the reactants runs out.

  The reactant that runs out first is known as the limiting reactant.

  We determine the limiting reactant by determining which reactant produces the least amount of a specific product.

Class Example   You run the Habor’s reaction:

N2 (g) + 3 H2 (g) 2NH3 (g)   How many moles of NH3 can be produced from 3.0 mol of N2

and 6.0 mol of H2?

Table Talk   The reaction:

2 H2 (g) + O2 (g) 2 H2O (g)   is used to produce electricity in hydrogen fuel cells. Suppose a

fuel cell contains 150. g of H2 and 1500. g of O2. How many grams of water can form?

Model Cards   On the handout, provide an answer to the problem that you are

working out and the reasoning behind your answer.   Complete all the problems, ask your table partners if you need

help on the reasoning!

  Read 3.4, 3.5, 3.6, and 3.7   Complete homework