Name: ______ANSWER KEY______Date: ______Period: ______

Stoichiometry - Expanding the Mole Idea Even More

Objective

To extend the mole concept and its calculations from elements and compounds to chemical reactions. To predict how much reactant is needed or how much product is produced in a given chemical reaction.

Stoichi-what?

1. Stoichiometry tells us the _mole_ relationships between reactants and products in a chemical reaction.

2. This is a very useful thing to know if you are trying to make a pharmaceutical drug, and you need to calculate how much starting materials you need. In this sense, stoichiometry can be thought of as a recipe.

An Analogy-Recipes


The following is a recipe for French toast. Use it to answer the questions that follow.

  1. This recipe makes six slices of French toast. If you were planning to make twelve slices of French toast, how much of each ingredient would you need to use?

6 eggs, 2 cup milk, 1 tsp salt, 12 slices bread, 4 tbsp sugar

  1. Explain how you determined your answer to #1.

The recipe makes six – you want 12, 12 is double 6, so double all the ingredients

  1. Determine the ratio of eggs to bread in the ingredients list. Did this ratio change when you made twelve slices of bread? Explain.

3 eggs: 6 breadthe numbers increased but the ratio is the same if simplified.

  1. Ratios are also important in balanced chemical equations. What is the ratio of hydrogen (H2) to oxygen (O2) in the following balanced equation? 2H2 + O2 2H2O 2: 1
  1. This ratio is the __molar_ ratio, and it is determined by using the _coefficients_ in front of each chemical.
  2. This ratio can be thought of in terms of moles or molecules, but NOT mass. For example, in theequation in #4 above, for every 2 molecules of hydrogen, you would expect to get_1 molecule_ of water. For every 2 moles of hydrogen, you would expect to get _1 mole_ of water.

Molar Ratio Review

Balance the following equation, and answer the questions which follow.

______N2 + __3__H2 __2_NH3

a. Molar ratio between nitrogen and ammonia (NH3)? _1:2_

b. Molar ratio between hydrogen and ammonia (NH3)? _3:2_

c. If you have 2 moles of N2, how many moles of NH3 should you expect to get? __4 moles__

d. If you start with 6 molecules of N2, how many molecules of NH3 should you expect to get? _12_

Instructions for Stoichiometry Calculations

In stoichiometry calculations, you must always use the mole ratio to convert between chemicals.

Always begin by converting what is given to moles (unless it is already in these units).

Use dimensional analysis, and remember to include units with every number.So you don’t get lost

Stoichiometry Problems

Use dimensional analysis to answer each. Show all work, units, & round correctly. 1 (a.) and (b.) are shown as examples

1.5F2(g) + 2NH3(g) N2F4(g) + 6HF(g)

a.) How many moles of HF will be produced from 4.00 moles of NH3?

b.) How many grams of N2F4 will be produced from 100.0 g of F2?

c.) How many liters of fluorine gas (at STP) are needed to produce 60.0 g HF?

60.0 g HF x 1 mol HF x 5 mol F2 x 22.4 L F2 = 56.0 L F2

20.0059g 6 mol HF 1 mol F2

d.) How many molecules of NH3 are needed to produce 3.27 x 1024 molecules of N2F4?

3.27 x 1024 molec N2F4 x 1 mol N2F4 x 2 mol NH3 x 6.02 x 1023 molec NH3

6.02 x 1023 molec 1 mol N2F2 1 mol NH3

= 6.54x1024 molecules NH3

2.4Al(s) + 3O2(g) 2Al2O3(s)

a.) 7.8 g of Al is reacted to produce what mass (in grams) of aluminum oxide?

7.8 g Al x 1 mol Al x 2 mol Al2O3 x 101.964 g = 15 g Al2O3

26.982g 4 mol Al 1 mol Al2O3

b.) How many atoms of aluminum are needed to produce 4.39 x 1023 molecules of Al2O3?

4.39 x 1023 molecules x 1mol Al2O3 x 4 mol Al x 6.02 x 1023 molecules = 8.78 x 1023 molecs Al

6.02x1023 2 mol Al2O31 mol

c.) How many liters of oxygen gas are needed to produce 100.00 g aluminum oxide?

100.0 g Al2O3 x 1 mol Al2O3 x 3 mol O2 x 22.4 L O2 = 32.95 L O2

101.964 g 2 mol Al2O3 1 mol O2

d.) How many moles of aluminum are needed to react with 9 moles of oxygen gas?

9 mol O2 x 4 mol Al = 12 mol Al

3 mol O2

3.2Mg(s) + O2(g)  2MgO(s)

a.) How many liters of oxygen are needed for the combustion of 134 g of Mg? Assume the reaction is at STP. 134 g Mg x 1 mol Mg x 1 mol O2 x 22.4 L O2 = 61.75 L O2

24.305g 2 mol Mg 1 mol O2

b.) How many atoms of magnesium are needed to produce 4 moles of magnesium oxide?

4 mol MgO x 2 mol Mg x 6.02 x 1023 atom Mg = 2 x 1024 atoms Mg

2 mol MgO1 mol Mg

c.) If you wanted to produce 100.0 g of MgO, what mass (in grams) of magnesium metal would you need? 100.0 g MgO x 1 mol MgO x 2 mol Mg x 24.305g Mg = 60.30 g Mg

40.305g MgO 2 mol MgO 1 mol Mg

d.) How many moles of oxygen gas will it take to completely react with 8 moles of magnesium?

8 mol Mg x 1 mol O2 = 4 mol O2

2 mol Mg

4.In a spacecraft, the carbon dioxide exhaled by astronauts can be removed by its reaction with lithium hydroxide, according to the following equation: CO2(g) + 2LiOH(s)  Li2CO3(s) + H2O(l). How many moles of lithium hydroxide are required to react with 20 mol CO2, the average amount exhaled by a person each day? 20 mol CO2 x 2 mol LiOH = 40 mol LiOH

1 mol CO2

5. In photosynthesis, plants use the energy from the sun to produce glucose, C6H12O6, 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? The balanced equation is: 6CO2(g) + 6H2O(l)  C6H12O6(s) + 6O2(g).

3.00 mol H2O x 1 mol C6H12O6 x 180.1608g glucose = 90.1 g C6H12O6

6 mol H2O1 mol glucose

  1. 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). If a toothpaste manufacturer reacts 30.00 g of HF with enough tin, how many grams of SnF2 can they expect to get? 30.00 g HF x 1 mol HF x 1 mol SnF2 x 156.706 g SnF2 = 117.5 g SnF2

20.0059 g 2 mol HF 1 mol SnF2