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Chapter 7- Molecular Formulas

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Warm Up: Determine the name and molar mass of the following compounds.

NaOH

MnO2

H2SO4

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How did you do?

NaOH- 40.00 g/mol, sodium hydroxide

MnO2- 86.94 g/mol, Manganese (IV) oxide

H2SO4 98.09 g/mol, Sulfuric acid

Be sure you go back and review the basics…

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Molecular Formula Activity

This activity was designed to help you remember some of the content we learned last semester.

You will also be learning about formulas (empirical and molecular) and percent composition.

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The Activity1. Work in groups of 3 or 4.2. Be sure that every word of the

activity is read out loud…take turns.3. Every person needs to fill in their

sheet.4. The bags containing LEGO molecules

are on my desk, come get one when you are ready. Balances are out.

Warm Up- Write the formulas

1. Calcium hydroxide2. Carbon monoxide3. Iron (III) chloride4. Copper (II) nitrate5. Sulfur tetroxide

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Finish up the activity….

Questions so far??

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What is Percent Composition?

Two ways of describing the composition of a compound.

HCl1. How many atoms of each

element2. The mass of each element in the

compound (percentage).

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How do I calculate Percent Composition?

1. Calculate the molar mass

2. Mass of element x 100= % composition Total Molar Mass

3. Add all the %compositions to make sure they total 100%.

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Find the percent composition of each element in these compounds

Li3N

BaSO3

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BaSO3

1. Molar mass

Ba 1mol x 137.3g/mol = 137.3gS 1mol x 32.07g/mol = 32.07gO 3mol x 16.00g/mol = 48.00g

Total =217.37g

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BaSO3

Mass percent Ba137.3g x 100 = 63.16%217.37g

Mass percent S32.07g x 100 = 14.75%217.37g

Mass percent O48.00g x 100 = 22.08%217.37g

63.16% + 14.75% + 22.08% =99.99% Close enough!!

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Look at the Percent Comp WS

You can do these no problem right?

Look at the problems at the bottom!

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Once you determine the Percent composition you can do various

calculations

How much potassium can be obtained in the decomposition of 50.0 g of K2Cr2O7?

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How did we determine the chemical formula of your LEGO compound?We take a known amount of the sample and take

it apart.

All the pieces are collected and weighed.

This provides the mass of each type of element allowing us to determine the mass percent of each element.

We use this information to determine the formula.

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Example

Suppose we have a compound that contains the elements C, H and O.

Total mass = 0.2015 g. C = 0.0806g H = 0.01353 gO = 0.1074 g

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Convert grams to moles.(0.0806g C) x 1mol C = 0.0067mol C

12.01g

(0.01353g H) x 1mol H =0.01342mol H 1.008g H

(0.1074g O) x 1mol O = 0.006713 mol O 16.00g O

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Now, look for the smallest number and divide!0.0067mol C

0.01342mol H

0.006713 mol O

This usually results in whole numbers-that tell us the ratio of atoms in the compound

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Warm Up- Jan 11, 2013

Determine the percent composition of nitrogen in NO3.

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How did you do on % Comp HW?

1. K=24.74%Mn=34.76%O = 40.50%

3. Mg = 16.39%N = 18.89%O = 64.72%

5. Al=15.77%S= 28.11%O= 56.12%

6. 39.2g O2

7. 17.5g Fe 8. 109 g Ag

Questions??

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Empirical Formula

Symbols of the elements with subscripts showing the smallest whole-number mole ratio of the different atoms in the compound.

CH2O

1:2:1 Mole ratio

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Empirical vs. Molecular formulas

Empirical formula- smallest whole-number ratio of the atoms in a compound.

Molecular formula- gives the actual number of atoms in the compound.

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What would be the empirical formulas?

CH2O

C2H4O2

C3H6O3

C4H8O4

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Example: Step 4

An oxide of aluminum is formed by the reaction of 4.151g Al with 3.692g of Oxygen. Determine the empirical formula of the compound.

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Warm Up

If the empirical formula is CO, write 3 molecular formulas that would be correct.

Determining Molecular Formulas

If you know the empirical formula…CH2

And are given the molar mass of the molecular formula… 42.09g/mol

We want to figure out how many of the empirical formulas fit into the molecular formula.

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You try…

A compound has a molar mass of 150.05 g/mol. The compound has an empirical formula of NO. Determine the molecular formula.

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Sometimes you need to figure out the empirical formula first!A sample contains a compound that is 30.45% nitrogen and 69.55% Oxygen. The molar mass of the compound is 92.02g/mol. What is the molecular formula.

a. Assume 100g- makes % into gb. Convert grams to molesc. Divide by the smallest # molesd. Write the Empirical formula and determine the molar mass.e. Divide the molar mass of the compound by the molar mass of the empirical formula.

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Work on the WS…

Homework is the formula review…

Major quiz Wed/Thurs…

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Start Chemical Reactions…

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