Compound Stoichiometry
Compound Stoichiometry Unit 5
What is Stoichiometry?
Compound vs Reaction Stoichiometry Molecular Mass Hydrate problems Moles and Avagadro’s Number Percentage Composition Problems Empirical Formula Problems Molecular Formula Problems Hydrates again
Stoichiometry
Stoichiometry: Element measurement Stoichiometry problems involve calculations (you will need a calculator for this) 2 types of Stoichiometry Problems Compound Stoichiometry Reaction Stoichiometry
Types of Compound Stoichiometry Problems Formula/ Molecular Mass Problems
Mole Problems Avogadro’s Number Problems Percentage Composition Problems Empirical Formula Problems Molecular Formula Problems
Formula/Molecular Mass How to find Formula/Molecular Mass of a compound: write a correct formula for the compound
Round the average atomic mass (on periodic table) -round to the nearest whole numberMultiply the Rounded atomic mass of each element but the number of atoms of that element present (usually subscripted) Add the total rounded atomic masses of all elements in the compound and label it with amu: Atomic Mass Units
Ex 5-1: Formula/Molecular Mass Problems Find the formula mass of Calcium Phosphate: Ca3(PO4)2 Element # * AMU
AMU
Ca
3 * 40 =
120
P
2 * 31 =
62
O
8 * 16 =
128
Total AMU: 310 amu
Ex 5-2: Find the formula mass of ammonium sulfate: (NH4)2SO4
Ex 5-3: Find the Molecular Mass of dichloride heptoxide:
Hydrates Hydrates are ionic compounds which trap water molecules in their crystal structures Need to know how to name and recognize hydrates Formula: Ionic Compound * # H20
Naming: Name your Ionic Compound add a numerical prefix tot he word hydrate Ex: CuSO4 * 5 H2O pentahydrate
copper (II)
Hydrates Formula Mass
Add all the average atomic mass of the ionic compound together Add the mass of the # of H2O (18 amu) 5 H2O = 80 amu Add both the atomic masses of the ionic compound and H2O
Ex 5-5: Find the formula mass of barium chlorate hexahydrate Ba(ClO3)2 * 6 H2O
Moles A mole is a unit in chemistry that indicates a definite number of particles Just as a “dozen” is a universal unit in America for 12 a mole is the chemist’s universal unit for number of particles 1 mole = 6.022 * 1023 representative particles (Avogadro’s number)
What is a representative particle?
More Moles Elements (except diatomic) the representative particle is the Atom 1 mole of Aluminum = 6.022 *1023 Atoms of Aluminum For diatomic and compounds the representative particles is the Molecules 1 mole of CO2 = 6.022 * 1023 molecules of CO2 1 mole of O2 = 6.022 * 1023 molecules of O2
Even More Moles Ionic compounds the representative particles are formula units 1 mole NaCl = 6.022 * 1023 Formula Units Whats the point of all this? No matter what kind of compound and its overall weight it contains the same number of representative particles Periodic Table: the amu is the amount of grams to make 1 mole of that substance
H2O = 18 amu = 1 mole of H2O So H2O also has 18 g per 1 mole of H2O (helps when converting)
Ex 5-6: I have 200 grams of Calcium Carbonate, How many moles of Calcium Carbonate do I have:
What if I had 300 g of Calcium Carbonate: What if I had 50 g of Calcium Carbonate: What if I had 63.2 g of Calcium Carbonate:
Calculating Moles Tips 2 ways of calculating moles Formula: Moles = Mass of a substance (grams) / formula mass of substance Dimensional analysis
Ex 5-7: How many moles are in 28.7 grams of Lithium Nitrate LiNO3 = 69 g/mole By Equation: Mole = mass / mw
28.7 g / (69 g/mol)
.416 mol of LiNO3 Dimensional Analysis: 28.7 g
X
1 Mole 69 g
.416 mol LiNO3
Ex 5-8: How many Moles are in 100 g of water?
Ex 5-9: How many moles of iodine are there in 200 grams of Iodine? Hint: Iodine is a Diatomic
Ex 5-10: If 47.4 grams of an ionic compound are known to be 1.185 moles of that compound what is the formula mass of that compound?
Ex 5-11: If 256 grams of a pure monatomic element are known to be 8.26 moles of that element what element are we talking about?
Ex: 5-12: How many grams of water are in 1.34 moles of water?
Avogadro’s Number Problems: Avogadro’s Number ( 6.022 * 1023)
Represents particles in 1 mole of a substance non-diatomic element have 6.022 * 1023 atoms in 1 mole of that same element In diatomic molecules remember they are represented with 2 moles per element “set” so it would be 6.022 * 1023 atoms in 1 mole of that same element
Avogadro's Number Problems The atomic mass of 65 amu. This means that one mole of zinc weights 65 grams
There are also 6.022 * 1023 atoms of zinc in 65 grams (1 mole) of zinc.
Avogadro's Number Problems IF the atomic mass of water is 18 g/mole that means that there are 6.022 * 1023 molecules of water in 18 grams of it or 1 mole of it.
Avogadro's Number Problems Representative particles of Elements are represented in Atoms Representative particles of molecular compounds (2+nonmetals) are molecules
Representative particles of an ionic compound is called Formula units When in Doubt GO TO MOLES!!!
Ex 5-13: How many atoms are present in pure iron nail which weighs 13.2 g?
Ex 5-14: If there are known to be 8.24 *1026 molecules of water present, would the sample of water weigh more then a pound?
1lb = 454 g
Ex 5-15: How many moles of Carbon dioxide are present in 4.55 * 1024 molecules of CO2?
Ex 5-16: How many formula units are there in 7.55 grams of sodium sulfate?
Ex 5-17: If I know that a sample of mercury (II) chloride contains 7.32 * 1024 formula units how much does the sample weight in pounds?
1 lb = 454 g
Example 5-18: Which would contain more representative particles 46 grams of Sodium Hydroxide or 46 grams of Carbon Dioxide?
Example 5-19: Which weighs more 6.24 * 1023 formula units of sodium bromide or 6.24 * 1024 molecules of phosphorus tribromide?
Percent Composition Problems
You are comparing the parts to the hole numbers
Percent composition is base on mass not atoms
present We are going to find the percentage BY MASS of certain elements in a compound by comparing the mass of the element to the mass of the entire compound
How to find Percentage Composition 1.Write a correct formula for the compound in the question 2.Find the formula or molecular mass of the entire compound 3.Divide the total mass of the Element whose percentage you are looking for by the total mass of the entire compound 4.Multiply your result by 100 to convert to a percentage
5.Express your answer to 1 decimal place and add a
Example 5-20: What is the COMPLETE percentage composition (by mass) of Potassium Dichromate ?
Percentage of Each Element
Example 5-20 What is the percentage of Calcium in Calcium Phosphide?
Example 5-21: What is the percent of oxygen in barium chlorate hexahydrate?
Percentage Composition % s are excellent conversion factor since it represents a comparison of units to 100 units
The next problems can be worked using dimensional analysis or by finding the % of a particular element present and then multiplying it times the mass of the compound given
Example 5-22: How many grams of of Calcium are present in 156.8 g of chalk? (Calcium Carbonate)
Example 5-23: How many grams of pure iron can be recovered from 50 grams of iron ore which is known to contain 32% Iron (II) Nitrate?
Example 5-24:Lime stone is known to be 95% calcium carbonate. How many grams of pure calcium could you expect to obtain from a piece of lime stone weighing 100 kg?
Empirical Formula Problems Empirical Formula - shows the ratio of the smallest amount of atoms in a compound RATIO OF THE ATOMS PRESENT (CH2O) The Ratio tells us how many of each atom is present To solve empirical you must be given either the grams of each element present or the % of each element present
How to solve Empirical Formula Problems 1.Divide either the % of each element OR the Grams of each element Given by the atomic mass of the element. Leave your answer in 4 sig figs for accuracy 2.Compare the results you got in step 1 by selecting the smallest of the results and dividing all of the results by the smallest value. You should get a result which is very close to a whole number or very close to a mix number which contains a recognizable fraction ( 1/3 = .3333)
How to solve Empirical Formula Problems
3. If All results in #2 are whole number use them as subscripts and write the formula out. If all results in # 2 are not whole numbers you must multiply all the results in #2 by a number that will make all the numbers whole numbers (ex 1.5 * 2 = 3) 4.Write the metal first (if there is one) If there is a list of elements then list them in alphabetical order.
Example 5-25: A compound is found to contain 10.14% lithium, 20.29% nitrogen and 69.57% oxygen. What is the empirical formula of the compound?
Example 5-26 A 200 gram sample of a compound which contain only carbon, hydrogen and oxygen is found to contain: 94.74 grams of carbon, 21.05 grams of hydrogen, and 84.21 grams of Oxygen. What is the empirical formula of the compound?
Example 5-27: An organic compound is analyzed and found to contain 44.336% carbon, 6.388% hydrogen, 23.587% oxygen, and 25.799% nitrogen. What is the empirical Formula?
Molecular Formula Problems Molecular Formulas are this formulas that show the ACTUAL number of ever different atom present Not the ratio
Molecular formulas are not reduced even though they could be Must have for these problems: Molecular mass of the compound The empirical formula of the compound or the means to find the empirical formula
How to find the molecular formula: 1.Find the molecular mass of the empirical formula 2.Divide the given molecular mass by the molecular mass of the empirical formula. (The result should give you a whole number or a close to whole number ) 3.Use this whole number result to multiply all of the subscripts of the empirical formula by and write the molecular formula
Example 5-28: If the empirical formula for a carbohydrate is CH2O and its molecular mass is known to be 240 g/mol, what is the molecular formula for the carbohydrates?
Example 5-29: If the empirical formula for a hydrocarbon is known contain 92.3% carbon and the molecular mass is known to be 78 g/mole what is the molecular formula for the compound?
Hydrate Problems
Hydrates- compounds which trap water in their crystal structure Need to know before starting hydrate problems The mass of the Anhydrous compound (dry, dehydrated) The mass of the water which was trapped and driven off with heat
How to solve hydrate problems:
1.Divide the mass or percentage of the DRY compound by the formula mass of the ionic compound only (not the entire hydrate) and leave your answer in 4 sig figs 2.Divide the mass (or the percentage) of the water by the molecular mass of water which is always 18 g/mol, and leave your answer in at least 4 sig figs
How to Solve hydrate problems:
3.Compare the values you obtained in #1 and #2 by selecting the smaller of the two and dividing them both by it. (whole number or close) 1.This is the number of moles of water that the compounds traps 4.Write there formula of the ionic compound followed by a multiplication sign and then write the number of water molecules you just calculated
Example 5-30: A chemist heats a 300 gram sample of hydrated iron (III) nitrate until all the water of hydration has been driven off. The anhydrous (dry) compound is found to weight 179.7 grams. What is the correct name of the hydrate?
Example 5-31: A hydrate of Iron (II) sulfite is known to contian 44.26% water. What is the correct formula of the hydrate?
Example 5-32: When 500 grams of hydrated copper (II) sulfate were heated strongly in a crucible, 180.35 grams of water were released and the anhydrous copper (II) sulfate remained in the crucible. What is the correct formula for the hydrate?
What is Stoichiometry?
Compound vs Reaction Stoichiometry Molecular Mass Hydrate problems Moles and Avagadro’s Number Percentage Composition Problems Empirical Formula Problems Molecular Formula Problems Hydrates again
Stoichiometry
Stoichiometry: Element measurement Stoichiometry problems involve calculations (you will need a calculator for this) 2 types of Stoichiometry Problems Compound Stoichiometry Reaction Stoichiometry
Types of Compound Stoichiometry Problems Formula/ Molecular Mass Problems
Mole Problems Avogadro’s Number Problems Percentage Composition Problems Empirical Formula Problems Molecular Formula Problems
Formula/Molecular Mass How to find Formula/Molecular Mass of a compound: write a correct formula for the compound
Round the average atomic mass (on periodic table) -round to the nearest whole numberMultiply the Rounded atomic mass of each element but the number of atoms of that element present (usually subscripted) Add the total rounded atomic masses of all elements in the compound and label it with amu: Atomic Mass Units
Ex 5-1: Formula/Molecular Mass Problems Find the formula mass of Calcium Phosphate: Ca3(PO4)2 Element # * AMU
AMU
Ca
3 * 40 =
120
P
2 * 31 =
62
O
8 * 16 =
128
Total AMU: 310 amu
Ex 5-2: Find the formula mass of ammonium sulfate: (NH4)2SO4
Ex 5-3: Find the Molecular Mass of dichloride heptoxide:
Hydrates Hydrates are ionic compounds which trap water molecules in their crystal structures Need to know how to name and recognize hydrates Formula: Ionic Compound * # H20
Naming: Name your Ionic Compound add a numerical prefix tot he word hydrate Ex: CuSO4 * 5 H2O pentahydrate
copper (II)
Hydrates Formula Mass
Add all the average atomic mass of the ionic compound together Add the mass of the # of H2O (18 amu) 5 H2O = 80 amu Add both the atomic masses of the ionic compound and H2O
Ex 5-5: Find the formula mass of barium chlorate hexahydrate Ba(ClO3)2 * 6 H2O
Moles A mole is a unit in chemistry that indicates a definite number of particles Just as a “dozen” is a universal unit in America for 12 a mole is the chemist’s universal unit for number of particles 1 mole = 6.022 * 1023 representative particles (Avogadro’s number)
What is a representative particle?
More Moles Elements (except diatomic) the representative particle is the Atom 1 mole of Aluminum = 6.022 *1023 Atoms of Aluminum For diatomic and compounds the representative particles is the Molecules 1 mole of CO2 = 6.022 * 1023 molecules of CO2 1 mole of O2 = 6.022 * 1023 molecules of O2
Even More Moles Ionic compounds the representative particles are formula units 1 mole NaCl = 6.022 * 1023 Formula Units Whats the point of all this? No matter what kind of compound and its overall weight it contains the same number of representative particles Periodic Table: the amu is the amount of grams to make 1 mole of that substance
H2O = 18 amu = 1 mole of H2O So H2O also has 18 g per 1 mole of H2O (helps when converting)
Ex 5-6: I have 200 grams of Calcium Carbonate, How many moles of Calcium Carbonate do I have:
What if I had 300 g of Calcium Carbonate: What if I had 50 g of Calcium Carbonate: What if I had 63.2 g of Calcium Carbonate:
Calculating Moles Tips 2 ways of calculating moles Formula: Moles = Mass of a substance (grams) / formula mass of substance Dimensional analysis
Ex 5-7: How many moles are in 28.7 grams of Lithium Nitrate LiNO3 = 69 g/mole By Equation: Mole = mass / mw
28.7 g / (69 g/mol)
.416 mol of LiNO3 Dimensional Analysis: 28.7 g
X
1 Mole 69 g
.416 mol LiNO3
Ex 5-8: How many Moles are in 100 g of water?
Ex 5-9: How many moles of iodine are there in 200 grams of Iodine? Hint: Iodine is a Diatomic
Ex 5-10: If 47.4 grams of an ionic compound are known to be 1.185 moles of that compound what is the formula mass of that compound?
Ex 5-11: If 256 grams of a pure monatomic element are known to be 8.26 moles of that element what element are we talking about?
Ex: 5-12: How many grams of water are in 1.34 moles of water?
Avogadro’s Number Problems: Avogadro’s Number ( 6.022 * 1023)
Represents particles in 1 mole of a substance non-diatomic element have 6.022 * 1023 atoms in 1 mole of that same element In diatomic molecules remember they are represented with 2 moles per element “set” so it would be 6.022 * 1023 atoms in 1 mole of that same element
Avogadro's Number Problems The atomic mass of 65 amu. This means that one mole of zinc weights 65 grams
There are also 6.022 * 1023 atoms of zinc in 65 grams (1 mole) of zinc.
Avogadro's Number Problems IF the atomic mass of water is 18 g/mole that means that there are 6.022 * 1023 molecules of water in 18 grams of it or 1 mole of it.
Avogadro's Number Problems Representative particles of Elements are represented in Atoms Representative particles of molecular compounds (2+nonmetals) are molecules
Representative particles of an ionic compound is called Formula units When in Doubt GO TO MOLES!!!
Ex 5-13: How many atoms are present in pure iron nail which weighs 13.2 g?
Ex 5-14: If there are known to be 8.24 *1026 molecules of water present, would the sample of water weigh more then a pound?
1lb = 454 g
Ex 5-15: How many moles of Carbon dioxide are present in 4.55 * 1024 molecules of CO2?
Ex 5-16: How many formula units are there in 7.55 grams of sodium sulfate?
Ex 5-17: If I know that a sample of mercury (II) chloride contains 7.32 * 1024 formula units how much does the sample weight in pounds?
1 lb = 454 g
Example 5-18: Which would contain more representative particles 46 grams of Sodium Hydroxide or 46 grams of Carbon Dioxide?
Example 5-19: Which weighs more 6.24 * 1023 formula units of sodium bromide or 6.24 * 1024 molecules of phosphorus tribromide?
Percent Composition Problems
You are comparing the parts to the hole numbers
Percent composition is base on mass not atoms
present We are going to find the percentage BY MASS of certain elements in a compound by comparing the mass of the element to the mass of the entire compound
How to find Percentage Composition 1.Write a correct formula for the compound in the question 2.Find the formula or molecular mass of the entire compound 3.Divide the total mass of the Element whose percentage you are looking for by the total mass of the entire compound 4.Multiply your result by 100 to convert to a percentage
5.Express your answer to 1 decimal place and add a
Example 5-20: What is the COMPLETE percentage composition (by mass) of Potassium Dichromate ?
Percentage of Each Element
Example 5-20 What is the percentage of Calcium in Calcium Phosphide?
Example 5-21: What is the percent of oxygen in barium chlorate hexahydrate?
Percentage Composition % s are excellent conversion factor since it represents a comparison of units to 100 units
The next problems can be worked using dimensional analysis or by finding the % of a particular element present and then multiplying it times the mass of the compound given
Example 5-22: How many grams of of Calcium are present in 156.8 g of chalk? (Calcium Carbonate)
Example 5-23: How many grams of pure iron can be recovered from 50 grams of iron ore which is known to contain 32% Iron (II) Nitrate?
Example 5-24:Lime stone is known to be 95% calcium carbonate. How many grams of pure calcium could you expect to obtain from a piece of lime stone weighing 100 kg?
Empirical Formula Problems Empirical Formula - shows the ratio of the smallest amount of atoms in a compound RATIO OF THE ATOMS PRESENT (CH2O) The Ratio tells us how many of each atom is present To solve empirical you must be given either the grams of each element present or the % of each element present
How to solve Empirical Formula Problems 1.Divide either the % of each element OR the Grams of each element Given by the atomic mass of the element. Leave your answer in 4 sig figs for accuracy 2.Compare the results you got in step 1 by selecting the smallest of the results and dividing all of the results by the smallest value. You should get a result which is very close to a whole number or very close to a mix number which contains a recognizable fraction ( 1/3 = .3333)
How to solve Empirical Formula Problems
3. If All results in #2 are whole number use them as subscripts and write the formula out. If all results in # 2 are not whole numbers you must multiply all the results in #2 by a number that will make all the numbers whole numbers (ex 1.5 * 2 = 3) 4.Write the metal first (if there is one) If there is a list of elements then list them in alphabetical order.
Example 5-25: A compound is found to contain 10.14% lithium, 20.29% nitrogen and 69.57% oxygen. What is the empirical formula of the compound?
Example 5-26 A 200 gram sample of a compound which contain only carbon, hydrogen and oxygen is found to contain: 94.74 grams of carbon, 21.05 grams of hydrogen, and 84.21 grams of Oxygen. What is the empirical formula of the compound?
Example 5-27: An organic compound is analyzed and found to contain 44.336% carbon, 6.388% hydrogen, 23.587% oxygen, and 25.799% nitrogen. What is the empirical Formula?
Molecular Formula Problems Molecular Formulas are this formulas that show the ACTUAL number of ever different atom present Not the ratio
Molecular formulas are not reduced even though they could be Must have for these problems: Molecular mass of the compound The empirical formula of the compound or the means to find the empirical formula
How to find the molecular formula: 1.Find the molecular mass of the empirical formula 2.Divide the given molecular mass by the molecular mass of the empirical formula. (The result should give you a whole number or a close to whole number ) 3.Use this whole number result to multiply all of the subscripts of the empirical formula by and write the molecular formula
Example 5-28: If the empirical formula for a carbohydrate is CH2O and its molecular mass is known to be 240 g/mol, what is the molecular formula for the carbohydrates?
Example 5-29: If the empirical formula for a hydrocarbon is known contain 92.3% carbon and the molecular mass is known to be 78 g/mole what is the molecular formula for the compound?
Hydrate Problems
Hydrates- compounds which trap water in their crystal structure Need to know before starting hydrate problems The mass of the Anhydrous compound (dry, dehydrated) The mass of the water which was trapped and driven off with heat
How to solve hydrate problems:
1.Divide the mass or percentage of the DRY compound by the formula mass of the ionic compound only (not the entire hydrate) and leave your answer in 4 sig figs 2.Divide the mass (or the percentage) of the water by the molecular mass of water which is always 18 g/mol, and leave your answer in at least 4 sig figs
How to Solve hydrate problems:
3.Compare the values you obtained in #1 and #2 by selecting the smaller of the two and dividing them both by it. (whole number or close) 1.This is the number of moles of water that the compounds traps 4.Write there formula of the ionic compound followed by a multiplication sign and then write the number of water molecules you just calculated
Example 5-30: A chemist heats a 300 gram sample of hydrated iron (III) nitrate until all the water of hydration has been driven off. The anhydrous (dry) compound is found to weight 179.7 grams. What is the correct name of the hydrate?
Example 5-31: A hydrate of Iron (II) sulfite is known to contian 44.26% water. What is the correct formula of the hydrate?
Example 5-32: When 500 grams of hydrated copper (II) sulfate were heated strongly in a crucible, 180.35 grams of water were released and the anhydrous copper (II) sulfate remained in the crucible. What is the correct formula for the hydrate?
