Mass Relationships in Chemical Reactions

Chapter 3

Natural lithium is:

7.42% 6Li (6.015 amu)

92.58% 7Li (7.016 amu)

100= _______ amu

3.1

Average atomic mass of lithium:

The mole (mol) is the amount of a substance that contains as many elementary entities as there

are atoms in exactly ______ grams of 12C

3.2

1 mol = NA =____________________

___________________ number (NA)

Molar mass is the mass of 1 mole of in gramseggsshoes

marblesatoms

1 mole 12C atoms = ____________ atoms = ______ g

1 12C atom = 12.00 amu

1 mole 12C atoms = ___________ g 12C

1 mole lithium atoms = ___________ g of Li

For any element

___________ (amu) = __________ (grams)

3.2

One Mole of:

C S

Cu Fe

Hg

3.2

_____ g = ________________ amu

3.2

M = molar mass in g/mol

NA = Avogadro’s number

1 amu = ______________ g

Do You Understand Molar Mass?

How many atoms are in 0.551 g of potassium (K) ?

3.2

Molecular mass (or molecular weight) is the sum ofthe atomic masses (in amu) in a molecule.

SO2

1S amu2O amu

SO2 amu

For any molecule

________ mass (amu) = ______ (grams)

1 molecule SO2 = _________ amu

1 mole SO2 = ________ g SO2 3.3

Do You Understand Molecular Mass?

How many H atoms are in 72.5 g of C3H8O ?

3.3

Percent composition of an element in a compound =

n x molar mass of elementmolar mass of compound

x 100%

n is the number of moles of the element in 1 mole of the compound

C2H6O

3.5

Types of FormulasTypes of Formulas• ____________ Formula____________ Formula

The formula of a compound that The formula of a compound that expresses the _________________expresses the _________________ ratio ratio of of the atoms present.the atoms present.

Ionic formula are always ____________ Ionic formula are always ____________ formulaformula

• _______ Formula_______ Formula

The formula that states the The formula that states the __________ number of each kind of atom found in number of each kind of atom found in __________________ of the compound. of the compound.

To obtain an To obtain an Empirical Empirical FormulaFormula

1.1. Determine the _______________ of each Determine the _______________ of each element present, if necessary.element present, if necessary.

2.2. Calculate the number of Calculate the number of __________________ of of each element.each element.

3.3. Divide each by the smallest number of Divide each by the smallest number of moles to obtain themoles to obtain the_______________._______________.

4.4. If _____numbers are not obtainedIf _____numbers are not obtained** in step in step 3), multiply through by the _____ number 3), multiply through by the _____ number that will give all whole numbersthat will give all whole numbers

** Be careful! Do not round off numbers prematurelyBe careful! Do not round off numbers prematurely

A sample of a brown gas, a major air pollutant, A sample of a brown gas, a major air pollutant, is found to contain 2.34 g N and 5.34g O. is found to contain 2.34 g N and 5.34g O. Determine a formula for this substance.Determine a formula for this substance.

require require molemole ratios so convert grams to moles ratios so convert grams to moles

Calculation of the Molecular FormulaCalculation of the Molecular Formula

A compound has an empirical formula of NOA compound has an empirical formula of NO22. . The colourless liquid, used in rocket engines The colourless liquid, used in rocket engines has a molar mass of 92.0 g/mole. What is the has a molar mass of 92.0 g/mole. What is the molecular formula molecular formula of this substance?of this substance?

Empirical Formula from % Empirical Formula from % CompositionComposition

A substance has the following composition by mass: 60.80 % Na ; 28.60 % B ; 10.60 % H What is the empirical formula of the substance? Consider a sample size of _____ grams

This will contain: _____ grams of Na, ______ grams of B, and _____ grams H

Determine the number of ______ of eachDetermine the ________ number ratio

3.6

g CO2 mol CO2 mol C g C

g H2O mol H2O mol H g H

g of O = g of sample – (g of C + g of H)

Combust 11.5 g ethanol

Collect 22.0 g CO2 and 13.5 g H2O

6.0 g C = 0.5 mol C

1.5 g H = 1.5 mol H

4.0 g O = 0.25 mol O

Empirical formula C0.5H1.5O0.25

Divide by smallest subscript (0.25)

Empirical formula C2H6O

1. Write balanced chemical equation

2. Convert quantities of known substances into moles

3. Use coefficients in balanced equation to calculate the number of moles of the sought quantity

4. Convert moles of sought quantity into desired units

Mass Changes in Chemical Reactions

3.8

Other units

• Molarity– Moles solute / L solution

• Gases– 22.4 L = 1 mole of ANY GAS at STP

Methanol burns in air according to the equation

2CH3OH + 3O2 2CO2 + 4H2O

If 209 g of methanol are used up in the combustion, what mass of water is produced?

3.8

6 green used up6 red left over

Limiting Reagents

3.9

Method 1

• Pick A Product

• ?????

• The ____ answer will be the correct answer

• The _____that gives the _____answer will be the _______ reactant

Limiting Reactant: Method 1• 10.0g of aluminum reacts with 35.0 grams of chlorine gas

to produce aluminum chloride. Which reactant is limiting, which is in excess, and how much product is produced?

2 Al + 3 Cl2 2 AlCl3

• Start with Al:

• Now Cl2:

Method 2

• Convert one of the reactants to the _____

• See if there is enough _______ to use up the other ______

• If there is less than the ______amount, it is the ______ reactant

• Then, you can find the desired species

Do You Understand Limiting Reagents?

In one process, 124 g of Al are reacted with 601 g of Fe2O3

2Al + Fe2O3 Al2O3 + 2Fe

Calculate the mass of Al2O3 formed.

3.9

Use limiting reagent (Al) to calculate amount of product thatcan be formed.

g Al mol Al mol Al2O3 g Al2O3

3.9

Finding Excess Practice• 10.0g of aluminum reacts with 35.0 grams of

chlorine gas 2 Al + 3 Cl2 2 AlCl3

• We found that _______ is the limiting reactant, and _______ of aluminum chloride are produced.

Given amount of excess reactant

Amount of excess reactant actually used

Note that we started with the limiting reactant! Once you determine the LR, you should only start with it!

___________ is the amount of product that wouldresult if all the limiting reagent reacted.

_________ is the amount of product actually obtainedfrom a reaction.

% Yield = _________Yield

_________Yieldx 100

3.10