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Warm-Up. Write the formulas that go with the following names: Tricarbon Pentaoxide Osmium (III) Sulfide Magnesium Nitride Dinitrogen Tetraoxide. Chemistry: Empirical and Molecular Formulas. Unit Five, Day Six Kimrey 18 October 2012. Empirical Formula and Molecular Formula.

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Warm up
Warm-Up

  • Write the formulas that go with the following names:

    • TricarbonPentaoxide

    • Osmium (III) Sulfide

    • Magnesium Nitride

    • DinitrogenTetraoxide


Chemistry empirical and molecular formulas

Chemistry:Empirical and Molecular Formulas

Unit Five, Day Six

Kimrey

18 October 2012


Empirical formula and molecular formula
Empirical Formula and Molecular Formula

  • Empirical formula is the simplest chemical formula

    • Ex. CH4

  • Molecular formula is the chemical formula that can be reduced.

  • Molecular Formula- the formula for a compound in which the subscripts give the actual number of each element in the formulas it truly exists.

    • Ex. C2H8


Notice two things: 1. The molecular formula and the empirical formula can be identical. 2. You scale up from the empirical formula to the molecular formula by a whole number factor.


Calculating the empirical formula from the percent composition
Calculating the empirical formula from the percent composition

  • Steps

  • Assume the percent = grams.

  • Convert each amount of grams to moles.

  • Divide each number of moles by the smallest number of moles.

  • If all the numbers are not whole numbers, multiple everything by a number that will get you a whole number. (This may take a few tries!)

  • The whole numbers become subscripts in the formula.


  • Percent composition to mass

  • Mass to mole

  • Divide by small

  • Multiply ‘til whole


Example
Example composition

  • A compound is 32.38% sodium, 22.65% sulfur, and 44.99% oxygen. What is the empirical formula?


Example 2
Example 2 composition

  • A compound is 40.0% Carbon; 6.71% Hydrogen; 53.29% Oxygen. What is its empirical formula?


Practice
Practice composition

  • 89.94% C; the rest is H

  • 56.34% P; 43.66% O  

  • 43.64% P; 56.36% O  

  • 40.9% C; 4.58% H; 54.5% O  


Now finding the molecular formula
Now, finding the molecular formula composition

  • Find the empirical formula

  • Calculate the molar mass (formula mass)

  • Divide the given mass of the substance by the molar mass

  • Multiply each subscript of the empirical formula by the answer to #3


Example1
Example composition

  • You have 56.106 grams of a substance with an empirical formula of CH2. What is the molecular formula?


  • CuSO composition 4 ● 5H2O

  • MgSO4 ● 7H2O

  • SnCl2 ● 2H2O

  • Na2CO3 ● 10H2O


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