Percent Composition and Empirical Formulas Ch

Percent composition: percent, by mass, of each element in a compound Mass of element in 1 mol of compound Molar mass of compound % composition =x 100% Ex: Find percent composition of Cu 2 S 1.Find molar mass of Cu 2 S 159.2g 127.1g Cu x 100% = 79.85% Cu 159.2g Cu 2 S 32.1g S x100% = 20.2 % S 159.2g Cu 2 S

Ex: dinitrogen pentoxide is 25.9% nitrogen by mass. How many grams of nitrogen and oxygen are in a 125g sample of the compound?

Molecular Formula: total number of atoms of each lement in one molecule of a compound Empirical Formula: smallest whole number ratio of the elements in a compound Molecular or empirical? 1.C 2 H 4 2.CH 3.H 2 O 4.H 2 O 2 5.C 6 H 12 O 6 6.C 8 H 8 7.Na 2 SO 4 If given the molecular formula, find the empirical formula.

Find Empirical Formal from % Can go from % composition to empirical formula 1.change % to grams for each given (assume 100g sample) 2.change grams to mol (using molar mass from periodic table) 3.divide all by the lowest # mols (to get ratio) 4.write ratio as formula (if not whole #s, try multiplying all by 2) Ex: Quantitative analysis shows that a compound contains 32.38% sodium, 22.65% sulfur, and 44.99% oxygen. Find the empirical formula g Na, 22.65g S, 44.99g O g Na x 1mol/23.0g Na = 1.408mol Ns 22.65g S x 1mol/32.1g S = 0.706mol S 44.99g O x 1mol/16.0g O = 2.81 mol O Divide each by mol Na 1mol S 3.98 mol O Na 2 SO 4

Ex: A g sample of methyl benzoate, a compound used in the manufacture of perfumes, is found to contain g of carbon, g of hydrogen, and g of oxygen. What is the empirical formula of this substance? Answer: C 4 H 4 O

Ch 7.4 Pg 244 Practice: #1-3 Pg 249 section review # finish next class

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