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Empirical Formula
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Slide 1

Empirical Formula

Empirical Formula

From percentage to formula

Slide 2

The Empirical Formula

The Empirical Formula

The lowest whole number ratio of elements in a compound.

The molecular formula the actual ratio of elements in a compound

The two can be the same.

CH2 empirical formula

C2H4 molecular formula

C3H6 molecular formula

H2O both

Slide 3

Calculating Empirical

Calculating Empirical

Just find the lowest whole number ratio

C6H12O6

CH4N

It is not just the ratio of atoms, it is also the ratio of moles of atoms

In 1 mole of CO2 there 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

Slide 4

Calculating Empirical

Calculating Empirical

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.

Slide 5

Example

Example

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

Slide 6

3.220 mole C

3.220 mole C

16.09 mole H

3.219 mole N

C3.22H16.09N3.219

If we divide all of these by the smallest

one It will give us the empirical formula

Slide 7

Example

Example

The ratio is 3.220 mol C = 1 mol C 3.219 molN 1 mol N

The ratio is 16.09 mol H = 5 mol H 3.219 molN 1 mol N

C1H5N1 is the empirical formula

A compound is 43.64 % P and 56.36 % O. What is the empirical formula?

Slide 8

43.6 g P x 1mol P = 1.4 mole P 30.97 gP

43.6 g P x 1mol P = 1.4 mole P 30.97 gP

56.36 g O x 1mol O = 3.5 mole O 16 gO

P1.4O3.5

Slide 9

Divide both by the lowest one

Divide both by the lowest one

The ratio is 3.52 mol O = 2.5 mol O 1.42 mol P 1 mol P

P1.4O3.5

P1O2.5

Slide 10

Multiply the result to get rid of any fractions.

Multiply the result to get rid of any fractions.

P1O2.5

2 X

= P2O5

Slide 11

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