11Mr. Shields Regents Chemistry U04 L04

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Molar CompositionMolar CompositionWhen we see a chemical formula we immediately know howMany 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 molof C and 3 moles of oxygen atoms

And in 0.25 mol of NaHCO3 I can calculate that I have0.25 mol of Na, 0.25 mol of H, 0.25 mol of C and 0.75 mol of Oxygen atoms

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Back to HydratesBack to HydratesNow 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 If I had 1 mol1 mol of CoCl of CoCl22..6H6H22OO how many mol of waterhow many mol of water

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

Now, If I only had Now, If I only had 0.670.67 mole of CoCl mole of CoCl22..6H6H22O how O how

many mol of water would I have? many mol of water would I have?

CoClCoCl22..6H6H22OOHEATHEAT

CoClCoCl22H20H20

H20H20

H20H20

H20H20

H20H20

H20H20

++

0.67 x 6 = 4.02 mol

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Problem:

How many moles of water are in 3 molesof CaO∙8H20 ?

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?

Back to Hydrates – moles of Water

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

3 x 8 = 24 moles

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Moles of atoms in hydratesMoles of atoms in hydrates

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 Ans: 25 g of hydrate – 14 grams of remaining anhydrous Product = 11 grams of waterProduct = 11 grams of water

11g H11g H220 / 25g Hydrate = 44% water 0 / 25g Hydrate = 44% water

When a compound has NO water of hydration it is When a compound has NO water of hydration it is said to be Asaid to be Anhydrousnhydrous and is called an and is called an AnhydrideAnhydride. .

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HydratesHydrates

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

Answer:Answer: 15g - 9.5g = mass of the water = 15g - 9.5g = mass of the water = 5.5g5.5g

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

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Empirical formulaEmpirical formula

DEFINITION:

A chemical formula that states the simplest wholenumber ratios of atoms in a compound iscalled the EMPIRICAL FORMULA of the compound

Empirical formula’s ARE NOT NECESSARILY the sameas the molecular formula, but they Can be

Ex. C8H16 CH2

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Empirical formulaEmpirical 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

C2H4Cl2C3H3Br3

C2H6O

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Glucose is a carbohydrate important to sustaining life.Glucose is a carbohydrate important to sustaining life. It’s a compound with the molecular formula CIt’s a compound with the molecular formula C66HH1212OO66. .

glucose

What’s the Empirical formula?

Is it the same as itsMolecular formula?

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Empirical formulasEmpirical formulas

The molecular formula for Hydrogen Peroxideis H2O2. What’s the empirical formula?

The simplest whole number ratio of the elementsis HO. This is it’s empirical formula

What’s the empirical formulafor Water?

H2

O

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Molecular FormulaMolecular Formula

To determine this the Chemist needs one more pieceof information. They need the compound’s molar mass.

NOTE:

The chemist has several ways to determine MolarMass (Which we won’t go into) without knowing theActual molecular formula

Obviously a chemist needs to know whether an empiricalFormula is or is not also the molecular formula.

For example is water H20 or H4O2 or H6O3

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Let’s look at Benzene as an example

Molecular FormulaMolecular Formula

Benzene has an empirical formula of CH therefore the Molar mass is 13.02

If the chemist determines that the true molarmass is 78.12 can the empirical formula also bethe molecular formula?

NO, the molar masses do not match!

?!

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So what is the true molecular formula?Molecular FormulaMolecular Formula

If the true molar mass of Benzene is 78.12 and theMolar mass of the empirical formula is 13.02 howmuch greater is the molecular mass than the empiricalMass?

78.12/13.02 = 6

The molecular formula is 6x larger than the empiricalFormula. So the molecular formula must be 6x theEmpirical formula or C6H6

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Molecular FormulasMolecular FormulasLet’s try another problem.

A compound has an empirical formulaof CH2O. It’s Molar mass is determinedto 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