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Percent Composition, Empirical and Molecular Formulas

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Mass Percent Percent of each element a compound is composed of. Find the mass of each element, divide by the total mass of the compound, then multiply by a 100. –CH 4 –Al 2 (Cr 2 O 7 ) 3 –CaSO 4 · 2H 2 O

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Empirical Formula From percent composition, you can determine the empirical formula. Empirical Formula the lowest ratio of atoms in a molecule. Based on mole ratios.

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Steps for Calculating Empirical Formula 1. Calculate grams of each element. If % is given, assume 100 g sample. 2. Convert grams to moles of each element using Molar Mass.

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3. Divide by the lowest # of moles to get the Atomic Ratio. 4. Round to the nearest whole number. 5. Write the formula.

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Empirical Formula Example: A sample is 59.53% C, 5.38%H, 10.68%N, and 24.40%O what is its empirical formula.

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Rounding to the Nearest Whole Number: If the mole ratio comes out to be a fraction, multiply all ratios times the denominator of the fraction. Example: 2.48 = 2 ½ therefore 2.48 x 2 = 5

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Empirical Formula Example 2: A compound contains 26.56% Potassium, 35.41% chromium and the remainder is oxygen. Calculate the empirical formula.

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Molecular Formulas Not the simplest Ratio. To determine the molecular formula of a compound, you must be given the molecular mass.

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1. Determine the empirical formula. 2. Calculate the molar mass of the empirical formula 3. 4. Multiply all elements in the empirical formula by this factor.

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Example 1 Find the molecular formula of a compound that contains 30.45% nitrogen and 69.55 % oxygen. The molar mass of the compound is 92.02 g/mol.

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Example 2 Find the molecular formula of a compound that contains 56.36 g of oxygen and 43.64 g of phosphorus. The molar mass of the compound is 283.9 g/mol.

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