Percent Composition

1 / 18

# Percent Composition - PowerPoint PPT Presentation

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.

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.

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

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
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?
Method
• Write the mass (g) of each element in the compound. So….

25.9% N = 25.9g

74.1% O = 74.1g

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

 convert to moles

C = 60.0g/mol = 5

12.0g

H = 10.0g/mol = 10 1.0g

O = 32.0g/mol = 2

16.0g

 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