Empirical Formulas Definition Ex: A formula that gives the simplest whole number ratio of the different atoms in a compound. H 2 O 2 = hydrogen peroxide.

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Empirical and Molecular Formulas

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Presentation transcript:

Empirical Formulas Definition Ex: A formula that gives the simplest whole number ratio of the different atoms in a compound. H 2 O 2 = hydrogen peroxide HO = empirical formula for hydrogen peroxide C 6 H 12 O 6 = Glucose Empirical Formula? =

Empirical Formula Calculation Example: Empirical formulas are determined from % composition. The % composition by mass of a compound is 56.6% K, 8.7% C, and 34.7% O. Find it’s empirical formula

Empirical Formulas Steps 1. Assume 100g of the compound. (Change % to g) K = ______g C = ______g O = ______g 2. Convert g to moles. (HINT: Use dimensional analysis)

Empirical Formulas Show your DA! K = 1.45mol C =.72mol O = 2.17mol 3. Divide all moles by the smallest mole value. K = 1.45mol/.72mol = 2.0 C =.72mol/.72mol = 1.0 O = 2.17mol/.72mol = 3.0

Solve the following: 1. A compound is 80% C and 20% H. What is the empirical formula? (Follow the steps). 2. A compound was analyzed and found to contain 13.5g Ca, 10.8g O and.675g H. What is the empirical formula for the compound?

Solve the following: 3. A compound is 20.8% Magnesium, 42.7% Arsenic, and 36.5% O. What is the empirical formula? Write the name of the compound!

Molecular Formulas Definition Example: Show the types and numbers of atoms in a compound. (H 2 O 2 ) Same as the empirical formula or a whole # multiple of it. C 6 H 12 O 6 = molecular formula of glucose. CH 2 O= empirical formula of glucose (or any sugar) H 2 O = empirical and molecular formula of water

Molecular Formulas Needed: Need (or need to be able to calculate): Empirical Formula (you may need to find this ) Formula Mass or Molar Mass of Molecular Compound (must be given)

Molecular Formulas Solve The empirical formula of a compound is P 2 O 5 and it has a formula mass of g. What is the molecular formula of the compound?

Molecular Formulas- Steps 1. Molar mass of the molecular compound g (from problem) 2. Empirical formula of the compound. P 2 O 5 (from problem) 3. Molar mass of the empirical formula. 2(Mass of P) + 5(Mass of O) = g 4. Molar mass of molecular formula (must be the one given)/ molar mass of empirical formula (always the same or smaller #) g / g = Multiply subscripts in empirical formula by answer above 2(P 2 O 5 ) = P 4 O 10

Remember! By definition, the molecular formula is the same as or a whole number multiple of the empirical formula… If your number did not come out perfect- CHECK YOUR MATH!

Solve the following: 4. A compound has 7.6g Nitrogen, 17.4g Oxygen, and a molar mass of 138g. What is the empirical formula? What is the molecular formula? Write the name of the compound!