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

Mole to Mole conversions 2Al 2 O 3  Al + 3O 2 each time we use 2 moles of Al 2 O 3 we will also make 3 moles of O 2 2 moles Al 2 O 3 3 mole O 2 or 2 moles Al 2 O 3 3 mole O 2 These are the two possible conversion factors to use in the solution of the problem.

Practice: 2C 2 H O 2  4CO H 2 O If 3.84 moles of C 2 H 2 are burned, how many moles of O 2 are needed? (9.6 mol)

2C 2 H O 2  4CO H 2 O How many moles of C 2 H 2 are needed to produce 8.95 mole of H 2 O? (8.95 mol)

2C 2 H O 2  4CO H 2 O If 2.47 moles of C 2 H 2 are burned, how many moles of CO 2 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 + 3O 2  2Al 2 O 3

Al Al 2 O 3 m= 6.50 g m=? M= g/mol M= g/mol 2) Convert mass of Al to moles- n Al =m/M n Al = 6.50g 26.98g/mol = mol of Al

3) Set up mole ratio 2 mol Al 2 O 3 4 mol Al 4) Convert to moles of Al 2 O mol x 2 4 = mol of Al 2 O 3

5) Convert to mass of Al 2 O 3 m Al2O3 = n x M m Al2O3 = mol x g/mol m Al2O3 = g m= 12.3 grams of Al 2 O 3 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 10 3 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 mixed 2. Theoretical yield- what the balanced equation tells should be made 3. Percent yield 3. Percent yield = Actual Theoretical x 100

Example: 6.78 g of copper is produced when 3.92 g of Al are reacted with excess copper (II) sulfate. 2Al + 3 CuSO 4  Al 2 (SO 4 ) 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 Yield Percent 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) + O 2(g) → 2CO 2(g), what is the ratio of moles of oxygen used to moles of CO 2 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) + 3O 2(g) → 2Fe 2 O 3(s)

Which of the following is true for: C 5 H 12(l) + 8O 2(g) → 5CO 2(g) + 6H 2 O (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) + Al 2 O 3(s) ?

Which of the following is an incorrect interpretation of the equation 2S (s) + 3O 2(g) → 2SO 3(g) : a) 2 atoms S + 3 molecules O 2 → 2 molecules SO 3 or b) 2 g S + 3 g O 2 → 2 g SO 3 ?

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 Fe 2 O 3 to produce g Fe? Fe 2 O 3(s) + 3CO (g) → 3CO 2(g) + 2Fe (s)

If O 2 was the excess reagent, 8.30 mol of H 2 S were consumed, and g of water were collected, what is the percent yield of this reaction? 2H 2 S (g) + 3O 2(g) → 2SO 2(g) + 2H 2 O (g)

Volume-Volume Calculations: How many liters of CH 4 at STP are required to completely react with 17.5 L of O 2 ? CH 4 + 2O 2  CO 2 + 2H 2 O 17.5 L O L O2O2 1 mol O2O2 2 O2O2 1 CH 4 1 mol CH L CH 4 = 8.75 L CH L O 2 1 mol O 2 1 mol CH L CH 4 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 particles You 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 STP

Shortcut for Volume-Volume? How many liters of CH 4 at STP are required to completely react with 17.5 L of O 2 ? CH 4 + 2O 2  CO 2 + 2H 2 O 17.5 L O2O2 2 L O 2 1 L CH 4 = 8.75 L CH 4 Note: This only works for Volume-Volume problems.

Remember! Significant Digits Start with a balanced equation Be 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 + 3O 2  2Al 2 O 3 particles of Al 2 O 3 = ? mol of Al 2 O x 6.02 x =