Stoichiometry

Analogy: Recipe to make PB&J sandwiches

2 slices of bread + 1 tbsp of peanut butter(pb) + 1 tbsp of jelly PB&J sandwich

The ratios of ingredients are:

2slices bread/1tbsp pb 2 slices of bread/1 tbsp jelly

1tbsp pb: 1tbsp jelly2 slices of bread/1 sandwich

1tbsp pb/1sandwich1 tbsp jelly: 1 sandwich

2 slices of bread/1 tbsp jelly2 slices of bread/ 1 tbsp pb

Typical question: If I had 12 slices of bread, how many tbsp pb would I need? How many sandwiches could I make?

Study of the ______of reactants and products in a chemical reaction

You must have a ______chemical equation in order to solve the problem.

From the balanced chemical equation, you need to determine the ______ratios of the substances. A mole ratio is a ______factor that relates the number of moles of any 2 substances.

The ______in front of the chemical formula is equal to the number of moles of that substance

What are the mole ratios from the following equation? Mg + 2 HCl  2MgCl2 + H2

Mole ratios are conversion factors that allow you to determine how much reactant you need in a reaction or how much product you produce in a reaction.

Don’t forget that 1 mole of a substance = molar mass of a substance.

All reactions are theoretical under ideal conditions.

Four types of problems:

1. Mole-Mole (1 step problem)

  • Use the mole ratio only

Examples: 2 C2H2 + 5 O2 4 CO2 + 2H2O

  1. How many moles of O2 are needed to burn 3.84 moles C2H2?

b. How many moles of C2H2 are needed to produce 8.95 moles H2O?

c. If 2.47 moles C2H2 are burned, how many moles of CO2 are formed?

2. Mole-Mass (2 step problem)

  • Use the mole ratio and molar mass conversion factor

Examples: 2 NaN3  2 Na + 3N2

  1. If .500 moles NaN3react, what mass in grams of N2would result?

3. Mass-Mol (2 step problem)

  • Use the molar mass conversion factor and thenthe mole ratio

Examples: 2 Fe + 3CuSO4  Fe2SO4 + 3Cu

  1. If 10.1 grams Fe were added to a solution of CuSO4, how much solid copper would form in moles?

4. Mass-Mass (3 step problem)

  • Use the molar mass conversion factor, thenthe mole ratio and then the molar mass conversion factor

Examples: 2 Mg + SiCl4 2MgCl2 + Si

  1. How many grams of Mg are needed to make 9.3 grams of Si?

b. How many grams of SiCl4 are needed to make 9.3 grams Si?

c. How many grams of MgCl2 are produced along with 9.3 g Si?

Percent Yield

A yield is a ______.

There are 3 types of yields:

  1. Actual Yield (A) : the amount of product that will form when ACTUALLY doing the lab
  2. Theoretical Yield (T) : the amount of product that will form using the balanced chemical equation
  3. Percent Yield: the ratio of the actual yield compared to the theoretical yield

%yield = A/T x 100

Examples:

What is the percent yield of the reaction if 3.47gCu is made when 1.87gAl is reacted with CuSO4 ? 2 Al + 3CuSO4  Al2(SO4)3 + 3Cu

A=

T=

% yield=

Stoichiometry of Gases

Similar method from above but you need to use a different conversion factor

Molar volume states that 1 mole of any ideal gas will occupy 22.4 L of volume at STP 1 mole gas = 22.4L at STP

STP stands for (standard temperature & pressure) and is equal to the values of 0C and 1 atm pressure

3 types of problems:

1. Moles of gas to volume at STP (1 step)

  • Use the molar volume conversion factor
  1. A sample of nitrogen gas, N2 has a volume of 1.75 L at STP. How many moles of N2 are present?

b. What volume at STP will be occupied by .0035 mol CH4?

2. Mass of gas to volume of gas at STP (2 step)

  • Use the molar mass conversion factor, then molar volume conversion factor

Suppose you need 4.22 g Cl2 gas. What volume at STP would you use?

3. Mass of solid to volume of gas at STP (3 step)

  • Use the molar mass conversion factor, the mole ratio and then molar volume conversion factor

CaCO3(s)  CO2(g) + CaO(s)

Calculate the volume of CO2 at STP produced from the decomposition of 152 grams CaCO3 .

**A different type of gas stoichiometry problem :

  • Law of Combining Volumes states that at constant temperature and pressure, the volumes of gases can be expressed as whole number ratios

2H2 + O2  2H2O

2volumes + 1 volume  2 volumes

2 moles + 1 mole  2 moles

Example

How many liters of O3 are formed from 12L O2? Gases are at the same temperature and pressure. 3 O2  2 O3