Gravimetric Stoichiometry

• Is used to calculate masses of reactants and products in a reaction.

Mole to Mole conversions 2Al2O3 ® 4Al + 3O2

each time we use 2 moles of Al2O3 we will also make 3 moles of O2

2 moles Al2O3

3 mole O2

or2 moles Al2O3

3 mole O2

These are the two possible conversion factors to use in the solution of the problem.

Practice: 2C2H2 + 5 O2 ® 4CO2 + 2 H2O

If 3.84 moles of C2H2 are burned, how many moles of O2 are needed?

(9.6 mol)

2C2H2 + 5 O2 ® 4CO2 + 2 H2OHow many moles of C2H2 are needed to

produce 8.95 mole of H2O?

(8.95 mol)

2C2H2 + 5 O2 ® 4CO2 + 2 H2OIf 2.47 moles of C2H2 are burned,

how many moles of CO2 are formed?

(4.94 mol)

What did you notice when determining your mole ratio…..?

The one that you are looking for is always on top!

Practice makes perfect!

Steps to Calculate Stoichiometric Problems

1. Correctly balance the equation.

2. Convert the given amount into moles.

3. Set up mole ratios.4. Use mole ratios to calculate

moles of desired chemical.5. Convert moles back into final

unit.

Mass-Mass Problem:

6.50 grams of aluminum reacts with an excess of oxygen. How many grams of aluminum oxide are formed?

1) Set up the balanced equation:

4Al + 3O2 2Al2O3

4Al + 3O2 2Al2O3 Al Al2O3

m= 6.50 g m=? M= 26.98 g/mol M=101.98

g/mol

2) Convert mass of Al to moles-nAl =m/M nAl= 6.50g 26.98g/mol

= 0.24092 mol of Al

3) Set up mole ratio

2 mol Al2O3

4 mol Al

4) Convert to moles of Al2O3

0.24092 mol x 2 4

= 0.12046 mol of Al2O3

5) Convert to mass of Al2O3

mAl2O3= n x M

mAl2O3= 0.12046 mol x 101.96 g/mol

mAl2O3= 12.282 g

m= 12.3 grams of Al2O3 are formed

(3 sig. digs)

Another example:Iron (III) oxide reacts with carbon monoxide to produce iron metal and carbon dioxide. What mass of iron (III) oxide is required to produce 1.00 x 103

grams of iron?

1. Write balanced chemical equation.

2. Calculate number of moles of _____.

3. Use mole ratio to determine number of moles of _______.

4. Convert your amount of moles from step 3 into mass in grams.

Now YOU try *S*Question 9 – 14 HANDOUT

What is Yield? Yield is the amount of product made in a

chemical reaction.There are three types:1. Actual yield- what you actually get in the lab

when the chemicals are mixed2. Theoretical yield- what the balanced

equation tells should be made3. Percent yield = Actual

Theoreticalx 100

Example:6.78 g of copper is produced when

3.92 g of Al are reacted with excess copper (II) sulfate. 2Al + 3 CuSO4 ® Al2(SO4)3 + 3Cu

What is the theoretical yield?

What is the actual yield?

What is the percent yield?

= 13.8 g Cu

= 6.78 g Cu

= 49.1 %

Details on YieldPercent yield tells us how “efficient” a reaction is.

Percent yield can not be bigger than 100 %.

Theoretical yield will always be larger than actual yield!Why? Due to impure reactants; competing

side reactions; loss of product in filtering or transferring between containers; measuring

1)  A reaction of magnesium metal and sodium fluoride takes place. If you start with 5.5 grams of sodium fluoride, how many grams of magnesium metal is needed?

  

2)If you react 20 grams of hydrochloric acid and sodium sulfate, how many grams of sulfuric acid will be produced?

In the following reaction:

2CO(g) + O2(g) → 2CO2(g),

what is the ratio of moles of oxygen used to moles of CO2 produced?

When iron rusts in air, iron (III) oxide is produced. How many moles of oxygen react with 2.4 mol of iron in the rusting reaction:

4Fe(s) + 3O2(g) → 2Fe2O3(s)

Which of the following is true for: C5H12(l) + 8O2(g) → 5CO2(g) + 6H2O(l)

a) 9 moles of reactants chemically change into 11 moles of product, or

b) 9 atoms of reactants chemically change into 11 atoms of product?

How many moles of aluminum react with 1.2 mol of FeO:

2Al(s) + 3FeO(s) → 3Fe(s) + Al2O3(s)?

Which of the following is an incorrect interpretation of the equation 2S(s) + 3O2(g) → 2SO3(g):

a) 2 atoms S + 3 molecules O2 → 2 molecules SO3

or b) 2 g S + 3 g O2 → 2 g SO3?

The calculation of quantities in chemical equations is called ____.

Which type of stoichiometric calculation does not require the use of the molar mass?

How many grams of CO are needed to react with an excess of Fe2O3 to produce 209.7 g Fe?

Fe2O3(s) + 3CO(g) → 3CO2(g) + 2Fe(s)

If O2 was the excess reagent, 8.30 mol of H2S were consumed, and 137.1 g of water were collected, what is the percent yield of this reaction?

2H2S(g) + 3O2(g) → 2SO2(g) + 2H2O(g)

Volume-Volume Calculations:How many liters of CH4 at STP are required

to completely react with 17.5 L of O2 ?

CH4 + 2O2 ® CO2 + 2H2O

17.5 L O2 22.4 L O2 1 mol O2

2 mol O2

1 mol CH4

1 mol CH4 22.4 L CH4

= 8.75 L CH4

22.4 L O2 1 mol O2

1 mol CH4 22.4 L CH4

Notice anything relating these two steps?

Avogadro told us:Equal volumes of gas, at the same temperature and pressure contain the same number of particles.

Moles are numbers of particlesYou can treat reactions as if they happen liters at a time, as long as you keep the temperature and pressure the same. 1 mole = 22.4 L @ STP

Shortcut for Volume-Volume?How many liters of CH4 at STP are required

to completely react with 17.5 L of O2?

CH4 + 2O2 ® CO2 + 2H2O

17.5 L O2 2 L O2

1 L CH4 = 8.75 L CH4

Note: This only works for Volume-Volume problems.

Remember!Significant DigitsStart with a balanced equationBe aware of what you’re starting with

(mol/g/particles/L)Be aware of what you’re looking for

(mol/g/particles/L)

What if we were asked for the number of particles produced?Ex:4Al + 3O2 2Al2O3

particles of Al2O3 = ?0.12046 mol of Al2O3

0.12046 x 6.02 x 1023

=