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 number-

Multiply 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

Ca 120

2 * 31 = P

3 * 40 =

O

62

128 8 * 16 =

# * AMU Element AMU

310 amu

Total 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 copper (II)

pentahydrate

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

By Equation:

Dimensional Analysis:

Mole = mass / mw 28.7 g / (69 g/mol)

.416 mol of LiNO3

LiNO3 = 69 g/mole

1 Mole

69 g

28.7 g X .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

% sign at the end

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?