Percent Composition of Compounds & Empirical Formulas

An Introduction to Analytical ChemistryPacket #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 EquationMass 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 #4Mass 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