# Percent Composition and Empirical Formula Third week Percent

• Slides: 11

Percent Composition and Empirical Formula Third week

Percent Composition % is based on the mass of an item ¡ That is Grams to the rest of you! ¡ The equation is: Mass of item X 100 Total Mass ¡ Molar mass may be used to calculate % composition. ¡

Answer ¡ ¡ ¡ 2 Cl = 2 (35. 5) = 71 amu 2 F = 2 (19) = 38 amu 1 C = 1 (12) = 12 amu Molar mass = 121 g/mol % comp or % mass of Cl in compound = (71/121)x 100=58. 68

What is the percent mass of oxygen in Ascorbic Acid?

Answer: ¡ ¡ ¡ 6 Carbons = 72 amu 8 Hydrogens = 8 amu 6 Oxygens = 96 amu Molar mass = 176 g/mol % composition = (96/176) X 100 = 54. 55

Empirical Formula ¡ ¡ ¡ Empirical Formula is the ratio of moles of the elements that are contained in a substance (ionic or molecular). OR is the lowest whole number ratio of a chemical formula. Remember: Chemical Formula gives the actual ratio of elements in a chemical.

Using % Composition to calculate Empirical Formula First- find the % composition of each element. ¡ Second- assume a sample of 100 grams ¡ Third- find the grams of each element ¡ Fourth- convert grams to moles ¡ Fifth- Calculate mole ratio. Divide all moles by the smallest number of mole to make the smallest equal to one. Round all to the closest whole number. ¡

Determining the molecular formula from the Empirical Formula You can determine the molecular formula if you know the empirical formula and the molar mass. ¡ Find the empirical molar mass. ¡ Divide it by the molar mass. ¡ Multiply the subscripts of the empirical formula by the quotient. ¡ Worksheet ¡