CChem Day 27

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Empirical and Molecular Formulas : 

Empirical and Molecular Formulas

Objectives : 

Objectives Compare empirical and molecular formulas. Determine the empirical formula of a compound given the mass % of each element in the compound or the mass of each element in the compound.

Empirical and Molecular Formulas : 

Empirical and Molecular Formulas Empirical formula shows the smallest whole-number ratio of atoms in a compound NaCl, CuSO4, H2O, CH2O

Empirical and Molecular Formulas : 

Empirical and Molecular Formulas Formula unit shows the simplest ratio of cations to anions in an ionic compound NaCl, CuSO4, Na2C2O4, NH4NO2

Empirical and Molecular Formulas : 

Empirical and Molecular Formulas Molecular formula shows actual numbers of atoms in a molecule Molecular formula could be a multiple of an empirical formula C6H12O6, NH3, H2O, C2H4O2

Molecular vs. Empirical : 

Molecular vs. Empirical Acetic Acid Formaldehyde Glucose

Molecular vs. Empirical Formulas : 

Molecular vs. Empirical Formulas

Mole Ratio : 

Mole Ratio Ratio or fraction that compares the moles of one substance to the moles of another substance. Mole ratios for water: 2 mol H 1 mol O 1 mol O 2 mol H

To find the empirical formula : 

To find the empirical formula Convert grams of each element to moles Reduce the ratio of moles to get whole numbers

Sample Problem 1 : 

Sample Problem 1 What is the empirical formula of an unknown compound that contains 30.4% nitrogen and 69.9% oxygen by mass?

Sample Problem 1 : 

Sample Problem 1 What is the empirical formula of an unknown compound that contains 30.4% nitrogen and 69.9% oxygen by mass? Figure on using a 100-g sample, so that will mean you have 30.4 g N and 69.9 g O

Sample Problem 1 : 

Sample Problem 1 What is the empirical formula of an unknown compound that contains 30.4% nitrogen and 69.9% oxygen by mass?

Sample Problem 1 : 

Sample Problem 1 What is the empirical formula of an unknown compound that contains 30.4% nitrogen and 69.9% oxygen by mass? Write %age as grams

Sample Problem 1 : 

Sample Problem 1 What is the empirical formula of an unknown compound that contains 30.4% nitrogen and 69.9% oxygen by mass? Divide by avg. atomic mass

Sample Problem 1 : 

Sample Problem 1 What is the empirical formula of an unknown compound that contains 30.4% nitrogen and 69.9% oxygen by mass? Calculate

Sample Problem 1 : 

Sample Problem 1 What is the empirical formula of an unknown compound that contains 30.4% nitrogen and 69.9% oxygen by mass? Divide all by the smallest number of moles

Sample Problem 1 : 

Sample Problem 1 What is the empirical formula of an unknown compound that contains 30.4% nitrogen and 69.9% oxygen by mass? Calculate

Sample Problem 1 : 

Sample Problem 1 What is the empirical formula of an unknown compound that contains 30.4% nitrogen and 69.9% oxygen by mass? NO2 Write the subscripts

Sample Problem 2 : 

Sample Problem 2 What is the empirical formula of an unknown compound that contains 4.37 g phosphorus and 5.63 g oxygen by mass? Since the problem gives you grams in the problem, you don’t have to worry about converting % to grams

Sample Problem 2 : 

Sample Problem 2 What is the empirical formula of an unknown compound that contains 4.37 g phosphorus and 5.63 g oxygen by mass? Start with the grams from the problem

Sample Problem 2 : 

Sample Problem 2 What is the empirical formula of an unknown compound that contains 4.37 g phosphorus and 5.63 g oxygen by mass? Divide by avg. atomic mass

Sample Problem 2 : 

Sample Problem 2 What is the empirical formula of an unknown compound that contains 4.37 g phosphorus and 5.63 g oxygen by mass? Calculate

Sample Problem 2 : 

Sample Problem 2 What is the empirical formula of an unknown compound that contains 4.37 g phosphorus and 5.63 g oxygen by mass? Divide all by the smallest number of moles

Sample Problem 2 : 

Sample Problem 2 What is the empirical formula of an unknown compound that contains 4.37 g phosphorus and 5.63 g oxygen by mass? Calculate

Sample Problem 2 : 

Sample Problem 2 What is the empirical formula of an unknown compound that contains 4.37 g phosphorus and 5.63 g oxygen by mass? 1.0 mol P Mole ratio is 2.5 mol O You can’t have a “half” in a mole ratio or formula

Sample Problem 2 : 

Sample Problem 2 What is the empirical formula of an unknown compound that contains 4.37 g phosphorus and 5.63 g oxygen by mass? 1.0 mol P x 2 = 2.0 mol P Mole ratio is 2.5 mol O x 2 = 5.0 mol O Multiply all parts by 2

Sample Problem 2 : 

Sample Problem 2 What is the empirical formula of an unknown compound that contains 4.37 g phosphorus and 5.63 g oxygen by mass? Write the subscripts P2O5

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