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Empirical Formula

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Empirical Formula

From percentage to formula

The formulas for compounds can be expressed as an empirical formula and as a molecular(true) formula.

Empirical Molecular (true)Name

CHC2H2 acetylene

CHC6H6benzene

CO2CO2carbon dioxide

CH2OC5H10O5ribose

- An empirical formula represents the simplest whole number ratio of the atoms in a compound.

- It is not just the ratio of atoms, it is also the ratio of moles of atoms
- In 1 mole of CO2there is 1 mole of carbon and 2 moles of oxygen
- In one molecule of CO2 there is 1 atom of C and 2 atoms of O

- The molecular formula is the true or actual ratio of the atoms in a compound.

A. What is the empirical formula for C4H8?

1) C2H4 2) CH2 3) CH

B. What is the empirical formula for C8H14?

1) C4H72) C6H12 3) C8H14

C. What is a molecular formula for CH2O?

1) CH2O2) C2H4O2 3) C3H6O3

If the molecular formula has 4 atoms of N, what is the molecular formula if SN is the empirical formula? Explain.

1) SN

2) SN4

3) S4N4

A. What is the empirical formula for C4H8?

2) CH2

B. What is the empirical formula for C8H14?

1) C4H7

C. What is a molecular formula for CH2O?

1) CH2O2) C2H4O2 3) C3H6O3

If the molecular formula has 4 atoms of N, what is the molecular formula if SN is the empirical formula? Explain.

3) S4N4

If the actual formula has 4 atoms of N, and S is related 1:1, then there must also be 4 atoms of S.

molar mass = a whole number = n

simplest mass

n = 1 molar mass = empirical mass

molecular formula = empirical formula

n = 2 molar mass = 2 x empirical mass

molecular formula =

2 x empirical formula

molecular formula = or > empirical formula

A compound has a formula mass of 176.0 and an empirical formula of C3H4O3. What is the molecular formula?

1) C3H4O3

2) C6H8O6

3) C9H12O9

A compound has a formula mass of 176.0 and an empirical formula of C3H4O3. What is the molecular formula?

2)C6H8O6

C3H4O3 = 88.0 g/EF

176.0 g = 2.00

88.0

If there are 192.0 g of O in the molecular formula, what is the true formula if the EF is C7H6O4?

1) C7H6O4

2) C14H12O8

3) C21H18O12

If there are 192.0 g of O in the molecular formula, what is the true formula if the EF is C7H6O4?

3) C21H18O12

192 g O = 3 x O4 or 3 x C7H6O4

64.0 g O in EF

- Pretend that you have a 100 gram sample of the compound.
- That is, change the % to grams.
- Convert the grams to mols for each element.
- Write the number of mols as a subscript in a chemical formula.
- Divide each number by the least number.
- Multiply the result to get rid of any fractions.

- Calculate the empirical formula of a compound composed of 38.67 % C, 16.22 % H, and 45.11 %N.
- Assume 100 g so
- 38.67 g C x 1mol C = 3.220 mole C 12.01 gC
- 16.22 g H x 1mol H = 16.09 mole H 1.01 gH
- 45.11 g N x 1mol N = 3.219 mole N 14.01 gN

- If we divide all of these by the smallest
- one it will give us the subscripts for the empirical formula
- 3.220 mol C = 1 3.219 mol N
- 16.09 mol H = 5 3.219 mol N
- 3.219 mole N = 1
3.219 mol N

- Empirical formula: CH5N

Aspirin is 60.0% C, 4.5 % H and 35.5 O. Calculate its simplest formula. In 100 g of aspirin, there are 60.0 g C, 4.5 g H, and 35.5 g O.

60.0 g C x ___________= ______ mol C

4.5 g H x ___________ = _______mol H

35.5 g O x ___________ = _______mol O

60.0 g C x 1 mol C= 5.00 mol C

12.0 g C

4.5 g H x 1 mol H = 4.5 mol H

1.01 g H

35.5 g O x 1mol O= 2.22 mol O

16.0 g O

Divide by the smallest # of moles.

5.00 mol C = ________________

______ mol O

4.5 mol H = ________________

______ mol O

2.22 mol O = ________________

______ mol O

Are are the results whole numbers?_____

Divide by the smallest # of moles.

5.00 mol C = ___2.25_=2 ¼ = 9/4

2.22 mol O

4.5 mol H = ___2.00__

2.22 mol O

2.22 mol O = ___1.00__

2.22 mol O

Are the results whole numbers? no

A fraction between 0.1 and 0.9 must not be rounded. Multiply all results by an integer to give whole numbers for subscripts.

(1/2) 0.5 x2 = 1

(1/3)0.333 x 3 = 1

(1/4)0.25 x4 = 1

(3/4)0.75 x 4 = 3

(1/5) 0.20 x 5 = 5

Multiply everything x 4 to clear the denominator

C: 9/4 mol C x 4 = 9 mol C

H: 2.0 mol Hx 4 = 8 mol H

O: 1.00 mol O x 4 = 4 mol O

Use the whole numbers of mols as the subscripts in the simplest formula

C9H8O4

- Worksheet C: #1-7