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## PowerPoint Slideshow about ' Percent Composition' - nyssa-tanner

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Percent Composition

- Can be calculated if given:

masses of elements in compound

OR

the chemical formula

Percent Composition

- Can be used to:

calculate the mass of elements in a compound

determine the empirical formula of a compound

determine the molecular formula of a compound

Empirical Formula

- shows the simplest mole ratio of the elements.
- CO is a 1:1 ratio of carbon to oxygen
- H2O is a 2:1 ratio
- CO2 is a 1:2 ratio
- Empirical formulas can’t be reduced.

Molecular Formula

- shows the actual number of atoms in a molecule.
- The molecular formula for hydrogen peroxide is H2O2. Its empirical formula would be HO.
- Often the molecular formula is the same as the empirical formula: H2O, CO2

Empirical?

- CH4O
- yes, cannot be reduced further
- C2H6
- no, empirical would be CH3
- C3H10O
- yes
- C6H6O2
- no. What would empirical be?
- C3H3O

Calculating Empirical Formulas

- A chemist with an unknown compound can easily figure out its percent composition, but it is much more meaningful to know its formula.
- EXAMPLE: What is the empirical formula for a compound that is 25.9% nitrogen and 74.1% oxygen?

2. Convert the mass of each element to moles.

- N = 25.9g = 1.85 mol

14.0g/mol

- O = 74.1g = 4.63 mol

16.0g/mol

Calculate the simplest whole number ratio by dividing the number of moles by the smallest number of moles.

1.85 : 4.63 = 1 : 2.5

1.85 1.85

(If the result is not within 0.1 of a whole number, multiply all numbers by a whole number)

2 ( 1 : 2.5) = 2 : 5

Write the empirical formula.

N2 O5

- For inorganic compounds, write the most positive element first.
- For organic compounds, write C first, H second and all others alphabetically.

A special present just for you……..

- Page 135, Problems #20 & 21

Molecular Formula

Given the empirical formula and the gram formula mass (gfm)

OR

Given the percent composition and the gram formula mass (gfm)

Example #1

Calculate the molecular formula for NaO having a gfm of 78g.

Determine the efm (empirical formula mass).

NaO = 23.0g + 16.0g = 39.0

- Divide the efm into the gfm.

78.0 = 2

39.0

- This is the conversion factor used to determine the molecular formula.

Na2O2

Example #2

Find the molecular formula for a compound having a composition of 58.8% C, 9.8% H and 31.4% O and a gmm of 102g/mol.

- Determine the mass of each component.

C = 102g/mol x 58.8% = 60.0g/mol

H = 102g/mol x 9.8% = 10.0g/mol

O = 102g/mol x 31.4% = 32.0g/mol

Use moles as subscripts for components of compound

C5H10O2

- Check the gmm of this compound…does it equal 102.0g/mol?
- 5(12.0) + 10(1.0) + 2(16.0) = 102.0g/mol
- YES!

And Now…..

- Oh Yeah! And there’s more…
- Page 136, Problems #22 & 23

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