Stoichiometry: Chapter 9

STOICHIOMETRYChapter 9Modern ChemistryS.Martinez – Spring 2011

Ch 9 Vocab – 8 words Lab # 6 & Lab # 7 Reports are due

Intro to Stoichiometry

Composition vs Reaction Stoichiometry Comp – mass relationship of elements in

compounds Reaction – mass relationship between

reactants and products in chem rxn

4 Types of Reaction-Stoichiometry Problems Type 1:

Given and unknown amounts are in moles Type 2:

Given amt in moles and unknown needs to be in grams

Type 3: Given amt in grams and unknown needs to

be in moles Type 4:

Given and unknown amts are in grams

Mole Ratio

Any of the reaction problems can be solved using ratios.

Mole ratio is derived from the coefficients in a balanced chemical equation.

Ex: Decomposition of aluminum oxide by electrolysis 2Al2O3 4Al + 3O2

Ex: Decomposition of aluminum oxide by electrolysis Determine the amount of Aluminum that

can be produced from 13.0 mol of aluminum oxide.Equati

ons2Al2O3

4Al + 3O2

Moles 2 moles 4 moles 3 moles

Molar mass

Grams

Molar Mass as a Ratio

Using the same problem, calculate the number of grams of aluminum equivalent to 26.0 moles of aluminum.

Section 9.2

Chemical Equations help make predictions about chemical reactions without having to run the reactions in the laboratory.

All of the reaction stoichiometric problems are theoretical, meaning we are assuming ideal conditions in which all reactants are converted into products.

Theoretical stoichiometric calculations show the maximum amt of product that could be obtained.

Problem Type #1

+Amt of given substance in mol

Amt of unknown substance in mol

In a spacecraft, the carbon dioxide exhaled by astronauts can be removed by its reaction with lithium hydroxide, LiOH.

How many moles of lithium hydroxide are required to react with 20 mol of carbon dioxide, the average amount exhaled by a person each day?

CO2 +2LiOH Li2CO3 + H2O

Answer: 40 moles of LiOH

Ammonia is widely used as a fertilizer and in many household cleaners. How many moles of ammonia, NH3, are produced when 6 mol of hydrogen gas react with an excess of nitrogen gas?

Answer: 4 moles of Ammonia

The decomposition of potassium chlorate is used as a source of oxygen in the laboratory. How many moles of potassium chlorate are needed to produce 15 mol of oxygen?

Answer: 10 mol KClO3

The elements Lithium and oxygen react explosively to form lithium oxide. How many moles of lithium oxide will form if 2 mol of lithium react? (synthesis rxn)

Answer: 1 mole of Lithium Oxide

The disinfectenct hydrogen peroxide, H2O2, decomposes to form water and oxygen gas. How many moles of oxygen will result from the decomposition of 5 mol of hydrogen peroxide?

Answer: 2.5 mol of oxygen

Problem Type 2

Starting in moles ending in grams

Moles to moles to grams

In photosynthesis, plants use 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 mol of water react with carbon dioxide?

In photosynthesis, plants use 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 mol of water react with carbon dioxide?

CO2 + H2O C6H12O6 + O2

In photosynthesis, plants use 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 mol of water react with carbon dioxide?

CO2 + H2O C6H12O6 + O2

6CO2 + 6H2O 1C6H12O6 + 6O2

In photosynthesis, plants use 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 mol of water react with carbon dioxide?

CO2 + H2O C6H12O6 + O2

6CO2 + 6H2O C6H12O6 + 6O2

3 mol ? grams

6CO2 + 6H2O C6H12O6 + 6O2

3.00 mol of H20 x (1 mol glucose/6 mol of water) = 0.500 mol of glucose

0.500 mol of glucose x (180.18 grams/mol) = 90.09 grams of glucose C – (6 x 12.01) = 72.06 H – (12 x 1.01) = 12.12 O – ( 6 x 16.00) = 96.00 Total molar mass = 180.18 g/mol

What mass of carbon dioxide, in grams, is needed to react with 3.00 mol of water in the above photosynthetic reaction?

6CO2 + 6H2O C6H12O6 + 6O2

6CO2 + 6H2O C6H12O6 + 6O2

6CO2 + 6H2O C6H12O6 + 6O2

3.00 mol of water x ( 6 mol of water/6 mol of CO2 ) = 3.00 mol of CO2

3.00 mol of CO2 x (44.01 grams/mole) = 132. grams of carbon dioxide

C – (1 x 12.01) = 12.01 O – (2 x 16.00) = 32.00 Molar mass = 44.01 grams/mole

When magnesium burns in the air, it combines with oxygen to form magnesium oxide. What mass, in grams, of magnesium oxide is produced from 2.00 mol of magnesium?

When magnesium burns in the air, it combines with oxygen to form magnesium oxide. What mass, in grams, of magnesium oxide is produced from 2.00 mol of magnesium?

2Mg + O2 2MgO

2Mg + O2 2MgO

When magnesium burns in the air, it combines with oxygen to form magnesium oxide. What mass, in grams, of magnesium oxide is produced from 2.00 mol of magnesium?

2Mg + O2 2MgO

2Mg + O2 2MgO 2.00 mol Mg x (2 mol of MgO/2 mol of

Mg) = 2.00 mol MgO 2.00 mol MgO x (40.31 grams/mole) =

80.6 grams

What mass in grams of oxygen combines with 2.00 mol of magnesium in this same reaction?

2Mg + O2 2MgO

What mass of glucose, C6H12O6 (molar mass 180.18 g/mol, solved from a previous problem) can be produced from a photosynthesis reaction that occurs using 10 mol CO2?

6CO2 + 6H2O C6H12O6 + 6O2

Answer: 300 grams of glucose

When sodium azide is activated in an automobile airbag, nitrogen gas and sodium are produced according to the equation:

NaN3 Na + N2

If 0.500 mol of NaN3 react, what mass in grams of Nitrogen would result?

Answer: 21.0 grams of Nitrogen

Carborundum, SiC, is a hard substance made by combining silicon dioxide with coke(C) as follows:

SiO2 + 3C SiC + 2CO

What mass in grams of SiC is formed from the complete reaction of 2.00 mol of carbon?

Answer: 26.7 grams of SiC

Coal can be converted to methane gas by a process called coal gasification. The equation for the reaction is the following:

2C + 2H2O CH4 + CO2

What mass in grams of carbon is required to react with water to form 1.00 mol of CH4?

Answer: 24.0 grams C

Problem Type 3

Starting in grams ending in moles

Grams to moles to moles

The 1st step in the industrial manufacture of nitric acid is the catalytic oxidation of ammonia.

NH3 + O2 NO + H2O The reaction is run using 824 g of

ammonia and excess oxygen. How many moles of nitrogen monoxide

are formed? How many moles of water are formed?

4NH3 + 5O2 4NO + 6H2O The reaction is run using 824 g of

ammonia and excess oxygen. How many moles of nitrogen monoxide

are formed? How many moles of water are formed?

48.3 mol NO 72.5 mol of water

Problem Type 4

Starting in grams ending in grams

Grams of given to moles of given to moles of unknown to grams of unknown

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 + 2HF SnF2 + H2

How many grams of SnF2 are produced from the rxn of 30.00 g of HF with Sn?

Sn + 2HF SnF2 + H2

Sn 2HF SnF2 H2

Moles

Given

Molar mass

Moles calc.

Mass calc.

117.5 g SnF2

Laughing gas (dinitrogen monoxide) is sometimes used as an anesthetic in dentistry. It is produced when ammonium nitrate is decomposed according to the following rxn.

NH4NO3 N2O + H2O If 360 g of ammonium nitrate are

decomposed how many grams of dinitrogen monoxide will be formed?