Unit 5 Moles and Stoichiometry Lesson 2: The Molar Relationships.

Unit 5

Moles and Stoichiometry

Lesson 2:The Molar Relationships

Molar Relationships

• We can convert between moles, NA, molar mass and volume using molar relationships and dimensional analysis.

• Relationships:

grams and moles

grams and representative particles

moles and representative particles

moles and liters

How to solve Molar Conversion Problem?

Molar conversions require the use of a problem solving process called dimensional analysis.

Method of calculation utilizing a knowledge of units.

Given units can be multiplied or divided by a factor to give the desired units.

How to solve Molar Conversion Problem?

Conversion factors are used to manipulate units:

Desired unit = given unit (conversion factor)

The conversion factors are simple ratiosos

unitgiven

unit desiredfactor Conversion

Conversion Factors

• ratio that comes from a statement of equality between 2 different units

dollarquarters 14

Example:

statement of equality

conversion factor

quarters

dollar

4

1

dollar

quarters

1

4or

Conversion Factors• can be multiplied by other numbers

without changing the value of the number

quartersdollar

quartersdollars 12

1

43

quarters

dollar

4

1

dollar

quarters

1

4or

How to solve Molar Conversion Problem?

In dimensional analysis always ask three questions:

1. What data are we given?

2. What quantity do we need?

3. What conversion factors are available to take us from what we are given to what we need?

The Four Steps to Solving Mole Problems

•  Step 1: Figure out how many parts your calculation will have by using the mole map diagram.

• Step 2: Make a T-chart, and put whatever information the problem gave you in the top left. After that, put the units of whatever you were given in the bottom right of the T, and the units of what you want to find in the top right.

The Four Steps to Solving Mole Problems

• Step 3: Put the conversion factors into the T-chart in front of the units on the right. Use your mole map diagram.

• Step 4: Cancel out the units from the top left and bottom right, then find the answer by multiplying all the numbers on the top together and dividing it by the numbers on the bottom.

Molar RelationshipsGrams and Moles

molar mass

(g/mol)

Mass in Grams

Moles

How many grams of lithium are in 3.50 moles of lithium?

3.50 mol Li

1

= g Li1 mol Li

6.94 g Li24.29

Converting Moles to Grams

Converting grams to moles

How many moles of lithium are in 18.2 grams of lithium?

18.2 g Li

1= mol Li

6.94 g Li

1 mol Li2.62

Converting Moles to Mass in a Compound

• How many grams of calcium chloride are in 0.98moles of calcium chloride?

.98mole CaCl2

110.98g CaCl2

1mole CaCl2

= 108.76 g CaCl21

Molar RelationshipRepresentative Particles and Moles

MOLES

6.02 1023

(particles/mol)

NUMBER

OF

PARTICLES

NUMBER

OF

PARTICLES

Converting Moles to Number of representative particles

1. How many molecules are in 2.50 moles of C12H22O11?

2.50 mol6.02 1023

molecules

1 mol= 1.51 1024

molecules C12H22O11

2. How many atoms of lithium are in 3.50 moles of lithium?

3.50 mol Li = atoms Li

1 mol Li

6.02 x 1023 atoms Li 2.11 x 1024

Calculate the moles of Aluminum Oxide in 2.0 x1024 Formula Units

2.0 x 1024

formula units

= 3.32 mole of Aluminum Oxide6.02 1023

Formula units

1 mole

Converting Number of RepresentativeParticles to mole

Molar RelationshipGrams and Representative Particles

molar mass

(g/mol)

MASS

IN

GRAMS

MOLESNUMBER

OF

PARTICLES

6.02 1023

(particles/mol)

Molecules to Grams

• Find the mass of 2.1 1024 formula units of NaHCO3.

2.1 1024

Formula units

1

1 mol NaHCO3.

6.02 1023

Formula units

= 293.06 g NaHCO3

2.9x102

84.01 g

1 mol

Grams to Molecules

• Find the number of formula units in 5 grams of MgSO4.

5 gramsMgSO4

1

1 molMgSO4

120.37g

= 2.50 1022 formula units MgSO4

6.02 1023

Formula units

1 mol

Molar RelationshipsVolume and Moles

Molar volume

(22.4 L/mol)

volume Moles

Avogadro’s Law

• equal volumes of gases at the same Temperature and Pressure contain equal numbers of molecules

• at same T and P, volumes varies directly with number of moles (n)

Molar Volume of Gases

Molar Volume of Gases

• like molar mass– mass of one mole of substance

• but with volume– volume of one mole of substance

• because of Avogadro’s law, one mole of any gas has the same volume as any other gas at the same T and P

Molar Volume of Gases• Standard Temperature and Pressure

– STP– 1 atm and 0°C

• Standard Molar Volume of Gas– volume of one mole of gas at 1 atm and 0°C

is 22.4L

• 22.4 L of any gas has one mole of particles but has different masses

A chemical reaction produces 0.0680 mol of oxygen gas.

What volume in liters is occupied by this gas sample at STP?

0.068 mol of O2

= 1.52L O2

1 mol O2

22.4L O2

Molar Volume Problems

Molar Volume Problems

• How many moles of Oxygen gas are in 112.00L Oxygen at STP?

112.00L

1 22.4L

1 mol= 5.0 mol Oxygen gas

Volume to Mass• A chemical reaction produced 98.0 mL of

sulfur dioxide gas at STP. What was the mass of gas made?

gmol

g

L

mol

mL

LmL 280.0

1

06.64

4.22

1

1000

10.98