Empirical Formulas Knowing exactly what it is. Percent Composition  Breaks down the amount of an element in a compound by percentage  Total mass of.

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Empirical & Molecular Formulas

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Presentation transcript:

Empirical Formulas Knowing exactly what it is

Percent Composition  Breaks down the amount of an element in a compound by percentage  Total mass of element / total mass of compound  Result will always be a decimal  Multiply by 100 to get the final percentage

Practice 1 What is the percent composition of the elements in NaC 2 H 3 O 2 ?

Practice 2 What is the percentage of Oxygen in NH 4 NO 3 ?

Practice 3 What is the percentage of Iron in Iron (III) Carbonate?

Empirical formulas  Formulas are represented in smallest whole number ratios  Derived from percent composition of a compound  Remember 100g = 100 % for easier calculations

Talk it Out… 1. Change percentage to grams assuming there are 100g of the sample 2. Divide by the molar mass of each element 3. Divide the moles of each element by the lowest number of moles of all the elements 4. Round to whole numbers to find ratios UNLESS there is.5, then multiply each result by 2 for whole number ratios

Problem 1 The percent composition of a compound was found to be 63.5% Silver, 8.2% Nitrogen, and 28.3% oxygen. Determine the compound’s empirical formula.

Problem 2 A g sample of an unidentified compound contains 29.84g of Sodium, 67.49g Chromium, and 72.67g Oxygen. What is the compounds empirical formula?

Problem 3 A 60.00g sample of an tetraethyl lead, a gasoline additive, contains 38.43g of Lead, 17.83g Carbon, and 3.74g Hydrogen. What is the compounds empirical formula?

Molecular Formula  You must have the empirical formula first  You must calculate the mass of the empirical formula  Don’t lose track of your numbers!

1. Calculate empirical formula 2. Calculate the mass of the empirical formula 3. Divide the given mass by the empirical mass, this will give you an element multiplier 4. Multiply your chemical quantities by the multiplier 5. Rewrite molecular formula with new chemical values

Problem 1 Determine the molecular formula of a compound with an empirical formula of NH 2 and a formula mass of amu.

Problem 2 The empirical formula of a hydrocarbon is found to be CH. Laboratory procedures have found that the molar mass of the compound is 78 g/mol. What is the formula of this compound?

Problem 3 A compound containing 50.05% Sulfur and 49.95% Oxygen has a molar mass of g/mol. Determine the empirical and molecular formula of this compound.