Warm Up• A molecule of sugar has 6

carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. What would be the SIMPLEST way to write the formula of sugar, while keeping the ratio of elements the same?

• WKSHT 8.2 out!

EQ: How do we determine the empirical and molecular formulas of compounds?HOT Q1: What is an “empirical formula?”HOT Q2: What is a “molecular formula”?HOT Q3: What is the incredibly catchy and clever rhyme used to remember empirical formulas?

Test Return/Tracking

• Averages:– 1st Block: 85.18% – 2nd Block: 89.69%– 3rd Block: 81.53%

‘Ray!

• http://www.youtube.com/watch?v=5L1tr0PIx20

• Track it. Track it. Track it.

CSI Chemistry Edition!

• At 6:02 am you and your team of medical examiners are called to the scene of crime. You find a dead body, covered in a mystery substance, but can find no other clues as to the murderer. Further analysis reveals the percentages of hydrogen, oxygen, and carbon in the substance. How could you determine the chemical formula of the substance in order to help identify the murderer?

Empirical Formula• When scientists create a substance in the

laboratory, they don’t know the chemical formula. BUT, based on how much of each element they put into they reaction they can come up with the ratio of elements in the compound to determine the chemical formula!

• There are 2 types of chemical formulas:– Empirical formula– Molecular formula

Notes: Empirical and Molecular Formulas

• Make a venn diagram!

Molecular

A molecular formula tells exactly how many of each atom are in the molecule.

molecular formula of sugar = C6H12O6

C C C C C C O

HH H

HH HH

H

H

H

H

H

OO

OO

O

An empirical formula tells the ratio of atoms in a molecule simplest form!

CH

HO: :

empirical formula of sugar = CH2O

Empirical

Take out Worksheet #3!

STEPS for finding Empirical Formula:

1) Percent to Mass: When percents are given, assume you have a 100 g sample (just change the % sign to a g)

2) Mass to moles: Convert grams to moles (use molar mass).

N-N-NOTES!

3) Divide by Small: Divide the moles of each element by the smallest mole. Divide ALL numbers by this value.

4) Multiply ‘till whole: If the decimal is close to .5, multiply by 2. If the decimal is close to .33 or .66, multiply by 3. Multiply ALL numbers.

I LOVE NOTES!

Practice1. Quantitative analysis shows that a compound contains

32.38% sodium, 22.65% sulfur, and 44.99% oxygen. Find the empirical formula of this compound.

Practice

2. A compound is analyzed and found to contain 36.70% potassium, 33.27% chlorine, and 30.03% oxygen. What is the empirical formula?

Practice3. Analysis of 20.0 g of a compound containing only calcium and

bromine indicates that 4.00 g of calcium are present. What is the empirical formula of the compound formed?

Molecular Formula

X (empirical formula) = molecular formula

1) To find x :molecular mass = x

mass of empirical formula2) Multiply the empirical formula’s subscripts by x to

get the molecular formula

Notes Time!

Practice

A 60.00 g sample of tetraethyl-lead, a gasoline additive, is found to contain 38.43 g lead, 17.83 g carbon, and 3.74 g hydrogen. Find its empirical formula. If the molecular formula has a mass of 180.00 g, find the molecular formula.

A 170.00 gram sample of an unidentified compound

contains 29.84 g sodium, 67.49 g chromium, and 72.67 g oxygen. What is the compound’s empirical formula? If the molar mass of the molecular formula is 680.00g, find the molecular formula.

Find the empirical formula of a compound found to

contain 26.56% potassium, 35.41% chromium, and the remainder oxygen. If the molecular formula has a mass of 1,470.00 grams, what is the molecular formula?

Practice!

• Finish worksheet 8.3

Exit Ticket

Percent Composition• The percent by mass of each element in the

compound.• Very simple calculations:

Total mass of element x 100% = % of elementTotal mass of compound

Steps for finding % composition:

1) Find molar mass (MM) the element(s) and the total MM of the compound.

2) Divide the mass of the element by the total mass of the compound and multiply by 100.

3) Check!! Make sure your %’s add up to 100.

Examples1. Find the % composition of oxygen in CaCO3.

2. Find the % composition of copper in copper (I) sulfide.

4. Zinc chloride, ZnCl2, is 50.02% chlorine by mass.a) What mass of chlorine is contained in 80.3 g of ZnCl2 ?b) How many moles of Cl is this?

5. Magnesium hydroxide is 54.87% oxygen by mass. a) How many grams of oxygen are in 175 g of the

compound?b) How many moles of oxygen is this?