Percent Composition of Compounds & Empirical Formulas

An Introduction to Analytical Chemistry

Packet #21

Introduction

• When utilizing a cellular phone, a limited amount of space is provided.

• Certain apps require more space than others.

• The amount of space used by a particular app can be defined as the percent of memory used by that app.

Percent Composition

• Percent composition is defined as a measure of how much of a given substance has been mixed with another substance. • The relative amounts of the

element in a compound

• The percent mass of each element in a compound.

Percent Composition Equation

• ((Mass of element present in 1 mole of compound) / (Molar Mass of compound)) * 100

Example #1

• Carvone is a substance that occurs in two forms, both of which have the same molecular formula (C10H14O) and molar mass. One type of carvone gives caraway seeds their characteristic smell; the other is responsible for the smell of spearmint oil. Compute the mass percent of each element in carvone.

Example #2

• Propane C3H8, the fuel commonly used in gas grills, is one of the compounds obtained from petroleum. Calculate the percent composition of propane. {Hint: -What is the mass percent of each element in propane?}

Percent Concentration

• When a solution is prepared in chemistry, the chemist is usually interested in the solution’s strength or concentration.

• Concentration is a measure of the quantity of solute dissolved to a given amount of solvent or solution.

Solution Solute

Solvent

Percent Concentration

• A more precise way of expressing concentration is to specify the quantity of solute that is dissolved in one hundred parts of the solution. • This expression is called

percent concentration.

Solution Solute

Solvent

Percent Concentration Equation Mass Percent Concentration

• ((Mass of solute) / (Mass of solution)) * 100

Example #3

• A student dissolves 25.0 grams of glucose in 475 grams of water. What is the mass percent concentration of the glucose solution?

Example #4 Mass Percent Composition

• When a 13.60g sample of a compound containing only magnesium and oxygen is decomposed, 5.40g of oxygen is obtained. What is the percent composition of this compound?

Example #5

• A compound is formed when 9.03g Mg combines with 3.48g N. What is the percent composition of this compound? (Hint: -What is the mass percent of all elements in the compound?)

Example #6

• When a 14.2g sample of mercury (II) oxide is decomposed into its elements by heating, 13.2g of Hg is obtained. What is the percent composition of this compound?

Example #7

• Calculate the percent nitrogen in these common fertilizers.

• NH3

• NH4NO3

EMPIRICAL FORMULA

Introduction I

• In an empirical formula it provides the lowest whole-number ratio of the atoms in a compound.

Introduction II

• Remember, in the empirical formula, the elements appear in smallest whole-number ratios.

• In the molecular formula H2O, the ratio of hydrogen to oxygen cannot be reduced further—hence it is also an empirical formula.

• H2nOn

• The empirical formula shows the smallest whole-number ratio of the atoms in the compound while the molecular formula tells the actual number of each kind of atom present in the compound.

Example #1

• A compound is analyzed and found to contain 25.9% nitrogen and 74.1% oxygen. What is the empirical formula of the compound?

Example #2

• Calculate the molecular formula of a compound whose molar mass is 60.0g/mol and empirical formula is CnH4nNn.

Example #3

• Find the molecular formula of ethylene glycol , which is used as antifreeze. The molar mass is 62g/mol and the empirical formula is CnH3nOn.

Example #4

• Which pair of molecules has the same empirical formula?

• C2H4O2; C6H12O6

• NaCrO4; Na2Cr2O7

Review