Stoichiometry

• The branch of chemistry that deals with the mass relationships of elements in compounds and the mass relationships between reactants and products in a chemical equation

• Stoichiometry is like following a recipe

• Focuses on mass relationships

* The key is a balanced equation and reading the equation in terms of…Coefficients!

Let’s try this

Na + Cl ______

1. Type of reaction?2. Skeleton equation3. Balanced equation: *Check for diatomic elements! *Check charges on ionic compounds! *Count the atoms and balance!

1. How is this read in terms of moles?2. What are all the possible mole

ratios?3. “RECIPE” for moles to moles, moles

to mass, mass to moles and mass to mass

Conversions of Quantities in Moles

• CO2 (g) + 2LiOH(s) → Li2CO3(s) + H2O (l)

How many moles of lithium hydroxide are required to react with 20 moles of carbon dioxide, the average amount exhaled by a person each day?

List the givens and unknowns!

2 moles/1mole = x/20moles

Answer = 40 moles

Do Now

• Practice Problems 1 and 2 on page 306

• Remember! Start with the skeleton equation and then balance the equation before calculating an answer!

The pain!The pain!

Conversions of Amounts in Moles to Mass

• Plants use energy from the sun to produce glucose and oxygen from the reaction of carbon dioxide and water. What mass, in grams, of glucose is produced when 3.00 moles of water react with carbon dioxide?

1) Write the skeleton equation.2) Write the balanced equation.3) List the given and unknown.4) Calculate the mole ratio.5) Multiply the mole ratio by the molar mass of the unknown.Answer: 90.1 grams

Question cont’d

• What mass of carbon dioxide, in grams, is needed to react with 3.00 moles of water?

More Problems

Do practice problems 1 and 2 on page 308

Conversions of Mass to Amounts in Moles

• The first step in the industrial manufacture of nitric acid is the catalytic oxidation of ammonia.

NH3 (g) + 02 (g) → NO (g) + H2O (g)

(unbalanced)

• This reaction is run using 824 g NH3 and excess oxygen.

1. How many moles of NO are formed?

2. How many moles of H2O are formed?

Solve Question 11. Balance the equation.2. List the given and unknown.3. Find the molar mass of NH3.

4. Find the mole ratio of NO to NH3.

5. Convert the given mass to moles of the given.6. Find the answer using the mole ratio.

4NH3 (g) + 502 (g) → 4NO (g) + 6H2O (g)Given: mass of NH3 = 824 gramsUnknown: moles of NO produced 1 mole of NH3 = 17.04 grams4:4 ratio is 1:1 ratio824/17.04 = 48.4 mol NH3

1:1 ratio, so 48.4 mol NO

Solve Question 21. Balance the equation.2. List the given and unknown.3. Find the molar mass of NH3.

4. Find the mole ratio of H2O to NH3.

5. Convert the given mass to moles of the given.6. Find the answer using the mole ratio.

4NH3 (g) + 502 (g) → 4NO (g) + 6H2O (g)Given: mass of NH3 = 824 gramsUnknown: moles of H2O produced 1 mole of NH3 = 17.04 grams6:4 ratio is 3:2 ratio824/17.04 = 48.4 mol NH3

[6/4 = x/48.4 or 3/2 = x/48.4] Answer = 72.6 moles

Mass-Mass Calculations• Tin(II) fluoride, SnF2 , is used in some

toothpastes. It is made by the reaction of tin with hydrogen fluoride according to the following equation.

Sn(s) + 2HF(g) → SnF2(s) + H2(g)

How many grams of SnF2 are produced from

the reaction of 30.00 g HF with Sn?

1. The equation is balanced so list the given and unknown.

2. Calculate the molar masses of HF and SnF2.

3. Calculate the mole ratio of HF to SnF2.4. Calculate moles of HF based on given grams.5. Use the mole ratio and plug in the calculated moles of HF.

6. Multiply the number of moles of SnF2 by the molar mass of SnF2.

Molar mass of HF = 20.01 g

Molar mass of SnF2 = 156.71 gMole ratio is 2:1HF: 30 g/20.01 g = 1.499 mol2/1 = 1.499/x, so x = .7495.7495 x 156.71 = 117.45 g, or 117.5 g Answer: 117.5 g

Limiting Reagent• Once a reactant is used up, there can be no more

products produced

• The substance that is used up first is called the limiting reagent = the reactant that limits the amount of the other reactant that can be combined and thus the product that is produced

• Excess reagent = the substance not used up completely• Look at Figure 5 on page 312 in the text; then look at

Sample Problems F and G (Pages 313-314)• Take Notes!

Percentage Yield• Theoretical yield = what you

calculate/expect under perfect conditions

• Actual Yield = what you get in the lab

• Percentage Yield = (AY/TY) x 100%

• Look at Sample Problem H (p. 317)