Chapter 11 The Mole

I. Measuring Matter A. Counting Particles Chemists needed a convenient method for counting the number of atoms in a sample of a substance The SI base unit, the mole (mol), is used to measure the amount of a substance (# atoms, molecules, formula units)

- # of carbon atoms in exactly 12g of pure carbon-12 - Represented as Avogadro’s number, x A mole of marbles would cover the Earth to a depth more than six kilometers

B. Converting Moles to Particles and Particles to Moles There are 6.02x10 23 particles in 1 mole Conversion Factor = 6.02x10 23 particles 1mole Example: How many particles are in 3.5 moles of sugar? You can use the inverse of this conversion factor to determine how many moles are in a given number of particles

II. Mass and the Mole A. The Mass of a Mole 12 g of carbon-12 is the standard mole equal to 6.02x10 23 particles Moles of different substances are not always going to equal 12 g The molar mass of an element equals the mass in grams of one mole of that element - Carbon has a mass of 12g in one mole

The mass of all elements have been established relative to the mass of carbon-12 - Therefore the molar mass of any element is equal to its atomic mass and has the units g/mol

B. Using Molar Mass The number of moles of an element can be converted to an equivalent mass and measure on a balance Conversion Factor: molar mass mole The inverse of this conversion factor can help you determine how many moles are in a given mass of substance

C. Using 2 Conversion Factors Converting the mass of a substance to a number of particles takes two conversion factors The opposite converts the number of particles to the mass in grams

III. Moles of Compounds A. Chemical Formulas and the Mole You can calculate how many moles of an element are in one mole of a compound Ex. Water is represented as H2O In one molecule of H2O there are - 2 Hydrogens (2 moles of Hydrogen) - 1 Oxygen (1 mole of Oxygen) The number of moles of the element that goes in the numerator of the conversion factor is the subscript for that element in the chemical formula

B. The Molar Mass of Compounds The mass of a mole (molar mass) of a compound equals the sum of the masses of every particle that makes up the compound

C. Converting Moles of a Compound to Mass To convert moles of a compound to mass, you must first calculate the molar mass of a compound by adding together its individual molar masses of the elements that make up the compound

D. Converting the Mass of a Compound to Moles Using the inverse of the molar mass to mole ratio, you can determine how many moles of a compound are present in a given mass of that compound

E. Converting the Mass of a Compound to Number of Particles Two conversion factors are needed to convert mass to a number of particles in a compound The number of particles must then be multiplied by the number of moles of an element in a formula unit of the compound to determine the number of atoms of an element are in the given mass of the compound

IV. Empirical and Molecular Formulas A. Percent Composition The percent by mass of each element in a compound mass of element mass of compound x 100 = percent by mass

The percent composition of a compound is always the same The general equation for calculating the percent by mass of any element is a compound is mass of element in 1 mol compound molar mass of compoundx 100

B. Empirical Formula If the identities of the elements in a compound are known as well as the percent compositions, the formula of the compound can be determined First you have to determine the smallest whole number ratio of the moles of the elements in the compound - This gives you the empirical formula

The empirical formula may or may not be the same as the molecular formula Ex.Molecular formulaEmpirical formula H 2 OH 2 O H 2 O 2 HO

If the percent composition is given, you can assume the total mass of the compound is g Ex. A compound has a percent composition of 40.05% S and 59.95% O - If the total mass of the compound is g, then there are g S and g O The mass of each element can be converted to moles

When the moles are not whole numbers, divide each by the smallest number

If the numbers are still not whole numbers, multiply the numbers by the smallest number that will produce a ratio of whole numbers 1.5 x 2 = 3 3 x 2 = 6 1 x 2 = 2

C. Molecular Formula Specifies the actual number of atoms of each element in one molecule or formula unit of the substance You can determine the molecular formula of the compound if the experimentally determined mass of the compound is given and compared to the empirical mass experimentally determined molar mass mass of empirical formula

The whole number produced by the previous equation is multiplied by the subscripts of the empirical formula to determine the correct subscripts of the molecular formula

V. The Formula for a Hydrate Solids in which water molecules are trapped A. Naming Hydrates The number of water molecules associated with each compound is portrayed in the formula Prefixes to the word hydrate tells you how many molecules of water are attached to the compound

B. Analyzing a Hydrate To find the number of water molecules in a hydrate, you need to heat the hydrate to drive off the water after recording the initial mass of the hydrate The mass of the hydrate is again recorded after the hydrate is heated

The mass of water is the difference between initial and final masses The molar masses of the compound and the water are converted to moles The moles of water are divided by the moles of compound to determine the number of water molecules