Your Name:
Leader:
Observer:
Presenter:
Recorder:
Date:
This is a Sequential Activity

## Group Activity: Stoichiometry

### Conservation of Mass at Work

• Definition
• The word  Stoichiometry comes from the Greek stoicheion, which means to measure the elements
• A good definition of the term’s meaning in the study of chemistry is the “quantitative study of reactants and products in a chemical reaction” (from Chemistry by Raymond Chang)
• Stoichiometry allows one to calculate how much of a given product a reaction is expected to produce based on how much of the reactants are available
• Given the mass, volume and density, or the number of moles of reactants, one can calculate the mass, volume (if the density is known) or moles of product
• Molar Ratios
•  Calculations using stoichiometry depend on the molar relationships in chemical equations; this is why a properly balanced chemical equation is so important
• A properly balanced chemical equation shows the molar ratios of each of the species present, whether they are reactants or products
• Take the combustion of propane as an example: C3H8 + 5O2 --> 3CO2 + 4H2O
• The ratios found in this equation are as follows:
1 mol propane:5 mol oxygen
(each mole of C3H8 requires five moles of O2 to burn completely)
1 mol propane:3 mol carbon dioxide
(each mole of completely burned C3H8 produces three moles of CO2)
1 mol propane:4 mol water
(each mole of completely burned C3H8 produces four moles of H2O)
5 mol oxygen:3 mol carbon dioxide
(for every five moles of O2 consumed, three moles of CO2 are produced)
5 mol oxygen:4 mol water
(for every five moles of O2 consumed, four moles of H2O are produced)
3 mol carbon dioxide:4 mol water
(for every three moles of CO2 produced, 4 moles of H2O are produced)
• Each of these ratios can be written in reverse order and can be thought of as a fraction or a conversion factor
• The key to using stoichiometry is to calculate using moles and to convert whatever information you are given into moles, whether it be mass or volume and density
• It is easy to calculate the number of moles needed or moles produced if the starting information is given in moles: just find the appropriate molar ratio and multiply by the moles in the starting information (be sure to write the ratio with the quantity you start with on the bottom of the ratio)
• It only requires one additional step to find the mass of a substance that is needed or produced given the number of moles of one of the substances: just convert the answer you find in moles into grams using the molar mass of the substance
• Given the mass of one of the substances you can find the mass of reactants needed or products produced by first converting the given mass into a number of moles, finding the number of moles of the other substance using the proper molar ratio, then converting that number of moles into grams

page break

• To add another layer of complexity, say you are given a volume of a substance and asked to find the volume needed/produced; for this type of problem you always need to know the density of both the substance whose volume is known and the substance whose volume is the answer to the problem
• This type of problem is solved by converting the volume into a mass using the formula m = D·V (make sure your units all match in this equation!); next find the number of moles; next use that number to find the desired number of moles using the proper molar ratio; next use the number of moles to find the mass; finally, use the mass to find the volume using formula V = m/D
• Theoretical Yield
•  The theoretical yield is the amount of a given product one would expect would be produced based solely on the molar ratios and the amount of each starting material
• The starting materials may be present in amounts that do not match the molar ratios in the chemical equation
• When this is the case one of the reactants is a limiting reactant (or limiting reagent) and the other reactant(s) is/are said to be in excess
• An excess reactant is one that is not completely used up when the reaction is complete
• The limiting reactant is the one that will be used up before any of the others
• Because of this, the moles of the limiting reactant are what determine the theoretical yield of a reaction
• To find out which reactant is the limiting reactant you use the molar ratios given in the balanced chemical equation
• Take the combustion of propane again: if you have 10 mol C3H8 and 1 mol O2 you know that oxygen is the limiting reactant because each mole of propane requires five moles of oxygen to burn
• If you are given masses, the problem is not any harder; you just have to convert the mass of each reactant into a number of moles and multiply that number by the proper ratio to find out how many moles of the other reactant are needed
• If you have more moles of O2 than this calculation shows you will need then C3H8 is the limiting reactant
• If you have fewer moles of O2 than this calculation shows you will need then C3H8 is the excess reactant and O2 is the limiting reactant
• To find the theoretical yield of a reaction, use the number of moles of the limiting reactant only in calculating the amount of a given product produced

page break

### Finding Molar Ratios

Balance each of the following chemical equations. Write down all the molar ratios that are in each equation. First, write the ratios for the first reactant and each of the other substance. Second, write the ratios for the second substance with each of the others, except the first one since you have already written it. Work your way through each equation in this manner.
1.     H2 +     O2 -->     H2O
2.     K +     H2O -->     KOH +     H2
3.     P4O10 +     H2O -->     H3PO4

### Calculating Moles Needed/Produced

The following equations are already balanced. You are given an amount in moles for one of the substances in the equation. Find out how many moles of each of the other substances you would need (reactants) or would be produced (products).
1. Mg(OH)2 --> MgO + H2O     1.5 mol MgO
2. Pb(NO3)2 + 2NaCl --> PbCl2 + 2NaNO3     3.56 mol NaCl
3. Cu + 2AgNO3 --> 2Ag + Cu(NO3)2     1.78 mol Ag
4. 2C + O2 --> 2CO     1 mol O2
5. 2KNO3 --> 2KNO2 + O2     1.3 x 103 mol KNO3

page break

### Calculating Mass Needed/Produced

The following equations are already balanced. You are given an amount in grams for one of the substances in the equation. Find out how the mass of each of the other substances you would need or would be produced. You will (obviously) need to find the molar masses of each of the reactants and products.
1. 2C + O2 --> 2CO     14 g C
2. 2KNO3 --> 2KNO2 + O2     29.8 g KNO2
3. 3Fe + 4H2O --> Fe3O4 + 4H2     167.54 g Fe

### Stoichiometry with Limiting Reagents

1. Titanium (IV) oxide (TiO2) is used as a pigment in paints and as a whitening and coating agent for paper. It can be made by reacting O2 with TiCl4:
TiCl4 + O2 --> TiO2 + 2Cl2

If 4.5 mol of TiCl4 react with 3.5 mol O2, identify both the limiting and the excess reactants. How many moles of the excess reactant would remain if all of the limiting reactant is used up? How many moles of each product will be formed if the reaction goes to completion?

##### Chemical Ratios (Stoichiometry)

The word Stoichiometry (stoy'-ki-AH-me-tree) comes from the Greek stoicheion, which means to measure the elements. It is a technique for finding out how much of a chemical product you will get from a chemical reaction. It can also be used to figure out how much of a starting material you need to get a certain amount of a chemical product. It is arguably one of the most important pieces of chemical knowledge.

It works like this. A balanced chemical reaction is used to figure out the mathematical relationships among the reactants and products. These relationships take the form of ratios. Take an equation used in the text at the beginning of this group activity: 2C + O2 2CO. This equation shows that there is a 2:1 molar ratio between C and O2. That is, it takes one mole of O2 to completely burn up two moles of C. This is a direct logical conclusion from the idea that it takes one whole molecule of O2 to react with two atoms of C. Similarly, there is a 1:1 molar ratio between C and CO and a 1:2 molar ratio between O2 and CO.

Answer the following questions using equations you balanced earlier in this activity. Use a separate sheet of paper to work out the solutions, showing all work. One is done as an example.

1. Given 10 mol C, how many moles of O2 are required to completely react with all of the C?
```            1 mol O2
10 mol C × ---------- = 5 mol O2
2 mol C```
2. Given 5.4 mol O2 how many moles of O3 can be produced?
3. How much ammonia can be manufactured from 1,000 mol N2 assuming you have enough H2?
1. How many moles of HCl do you need to completely react with 7.7 × 10-2 mol of zinc to make ZnCl2 and H2?
2. How many moles of fluorine gas are needed to make 2.5 mol of SF6 assuming no shortage of sulfur?
3. You have 4.6 mol NH3. How many moles of O2 do you need to completely react with all of the ammonia?
Homework Assignment: Stoichiometry
For this packet to be complete, print out one copy of the periodic table for each student.
Last updated: Aug 1, 2006        Home  |