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% Composition and Empirical and Molecular FormulasPowerPoint Presentation

% Composition and Empirical and Molecular Formulas

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Molar Composition

When we see a chemical formula we immediately know how

Many and what kind of atoms are in the compound.

But number of atoms can also represent # of moles of atoms

In the compound. For example…

In one mole of NaHCO3 I have 1 mol of Na, 1 mol of H, 1 mol

of C and 3 moles of oxygen atoms

And in 0.25 mol of NaHCO3 I can calculate that I have

0.25 mol of Na, 0.25 mol of H, 0.25 mol of C and 0.75 mol of

Oxygen atoms

Back to Hydrates

HEAT

Now try this problem…

How many moles of water are there in 0.67 mol of CoCl2.6H2O

Think of it this way…

If I had 1 mol of CoCl2.6H2Ohow many mol of water

molecules would I have if all the water was driven off?

H20

H20

CoCl2.6H2O

CoCl2

H20

+

H20

H20

H20

Now, If I only had 0.67 mole of CoCl2.6H2O how many mol of water would I have?

0.67 x 6 = 4.02 mol

Back to Hydrates – moles of Water

Problem:

How many moles of water are in 3 moles

of CaO∙8H20 ?

3 x 8 = 24 moles

Problem:

Two moles of water were driven off when a sample of

CaO∙8H20 was heated leaving the anhydrous salt behind.

How many moles of hydrate were originally present?

IF I had 1 mole of hydrate I would have 8 moles of water but

I only got 2 moles of water so I only had 2/8 = 0.25 moles

Moles of atoms in hydrates

When a compound has NO water of hydration it is said to be Anhydrous and is called an Anhydride.

Here’s another problem involving Hydrates…

A 25 gram sample of a hydrate is heated until all the water is driven off. The mass of the anhydrous product remaining was 14 grams. What is the % water in the hydrate?

Ans: 25 g of hydrate – 14 grams of remaining anhydrous

Product = 11 grams of water

11g H20 / 25g Hydrate = 44% water

Hydrates

Problem: if a 15g sample of a hydrate is heated until all the water of hydration is driven off. The mass of the anhydrous product remaining is 9.5 grams. What is the percent of water in the hydrate?

Answer: 15g - 9.5g = mass of the water = 5.5g

% water = (5.5g/15g) x 100 = 36.67%

Empirical formula

DEFINITION:

A chemical formula that states the simplest whole

number ratios of atoms in a compound is

called the EMPIRICAL FORMULA of the compound

Ex. C8H16 CH2

Empirical formula’s ARE NOT NECESSARILY the same

as themolecular formula, but they Can be

Empirical formula

Ex. C6H6 (Benzene) is NOT an empirical formula

it’s the true Molecular Formula. Why?

BUT Consider Methane (CH4)

It’s molecular formula is also it’s empirical formula. Why?

What are the empirical formula’s of the following? Is the

Empirical formula the same as the molecular formula

C2H4Cl2

C3H3Br3

C2H6O

Glucose is a carbohydrate important to sustaining life.

It’s a compound with the molecular formula C6H12O6.

What’s the

Empirical formula?

Is it the same as its

Molecular formula?

Empirical formulas

- The molecular formula for Hydrogen Peroxide
- is H2O2. What’s the empirical formula?
- The simplest whole number ratio of the elements
- is HO. This is it’s empirical formula

What’s the empirical formula

for Water?

H2O

Molecular Formula

Obviously a chemist needs to know whether an empirical

Formula is or is not also the molecular formula.

For example is water H20 or H4O2 or H6O3

To determine this the Chemist needs one more piece

of information. They need the compound’s molar mass.

NOTE:

The chemist has several ways to determine Molar

Mass (Which we won’t go into) without knowing the

Actual molecular formula

Molecular Formula

Let’s look at Benzene as an example

Benzene has an empirical formula of CH therefore the

Molar mass is 13.02

If the chemist determines that the true molar

mass is 78.12 can the empirical formula also be

the molecular formula?

NO, the molar masses do not match!

?!

Molecular Formula

So what is the true molecular formula?

If the true molar mass of Benzene is 78.12 and the

Molar mass of the empirical formula is 13.02 how

much greater is the molecular mass than the empirical

Mass?

78.12/13.02 = 6

The molecular formula is 6x larger than the empirical

Formula. So the molecular formula must be 6x the

Empirical formula or C6H6

Molecular Formulas

Let’s try another problem.

A compound has an empirical formula

of CH2O. It’s Molar mass is determined

to be 180g/mol.

What is the molecular formula?

Step 1: Empirical molar mass = 30g

Step 2: Divide molar mass of the empirical formula into

the actual molar mass. 180/30 = 6

Step 3: Multiply all atoms in the empirical formula by 6

This is the molecular formula

C6H12O6

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