# 3.1 - Counting By Weighing

• Objects do not need to have identical masses to be counted by weighing

• For purposes of counting, the objects behave as though they were all identical, as though they each actually had the average mass

• Since atoms are so small, we deal with samples of matter that contain huge numbers of atoms

• We determine the number of atoms in a given sample by finding its mass

# 3.2 - Atomic Masses

• The modern system of atomic masses, instituted in 1961, is based on carbon twelve as the standard

• The use of the mass spectrometer allows atoms or molecules to pass into a beam of high-speed electrons

• They knock electrons off the atoms or molecules being analyzed and charge them into positive ions

• The amount of path deflection for each ion depends on its mass

• A comparison of the positions where the ions hit the detector plate gives very accurate values of their relative masses

• The mass for each element is given in the table inside the front cover of the text

• The atomic mass we use for carbon is an average value reflecting the average isotopes composing it

# 3.3 - The Mole

• Mole: A unit of measure that has been established for use in counting atoms

• The SI definition of the mole is the amount of a substance that contains as many entities as there are in exactly 12 g. of carbon-12

• Avogadro’s number is 6.022 × 10^23

• One mole of anything is 6.022 × 10^23

• The mass of 1 mole of an element is equal to its atomic mass in grams

# 3.4 - Molar Mass

• A substance's molar mass is the mass in grams of 1 mole of the substance

• The molar mass of a known substance is obtained by summing the masses of component atoms

• Methane is a molecular compound

• Many substances are ionic- they contain simple ions or polyatomic ions

• Formula unit: Ionic compounds that do not contain molecules

# 3.5 - Percent Composition of Compounds

• Two common ways of describing the composition of a compound:

• In terms of numbers of its constituent atoms

• In terms of the percentages of its elements

• Carvone is a substance that occurs in two forms having different arrangements of the atoms but the same molecular formula and mass

• One type of carvone gives caraway seeds their characteristic smell and the other type is responsible for the smell of spearmint oil

• Although Fleming is commonly given credit for the discovery of penicillin, there is good evidence that penicillium mold extracts were used in the nineteenth century by Lord Joseph Lister to cure infections

# 3.6 - Determining the Formula of a Compound

• It is often determined by taking a weighted sample of the compound and either decomposing it into its component elements or reacting it to oxygen to produce substances

• Molecular formula (empirical formula): where n is an integer

• Numbers very close to whole numbers, such as 9.92 and 1.08, should be rounded to whole numbers. Numbers such as 2.25, 4.33, and 2.72 should not be rounded to whole numbers

• Determining Molecular Formula from Empirical formula:

• Obtain the empirical formula

• Compute the mass corresponding to the empirical formula

• Calculate the ratio

• When the empirical formula subscripts are multiplied by this integer, the molecular formula results

• Method Two

• Using the mass percentages and the molar mass, determine the mass of each element present in one mole of compound

• Determine the number of moles of each element present in one mole of compound

• The integers form the previous step represent the subscripts in the molecular formula

# 3.7 - Chemical Equations

• A chemical change involves a reorganization of the atoms in one or more substance

• In a chemical reaction, atoms are neither created nor destroyed

• Reactants and products must occur in numbers that give the same number of each type of atom among both the reactants and products

• The chemical equation for a reaction gives two important types of information: the nature of the reactants and products and the relative numbers of each

• The relative numbers of reactants and products in a reaction are indicated by the coefficients in the balanced equation

# 3.8 - Balancing Chemical Equations

• The principle that lies at the heart of the balancing process is that atoms are conserved in a chemical reaction

• It is important to recognize that the identities of the reactants and products of a reaction are determined by experimental observation

• Most chemical equations can be balanced by inspection, that is, by trial and error

• Chromate and dichromate compounds are carcinogens and should be handled very carefully

# 3.9 - Stoichiometric Calculations: Amounts of Reactants and Products

• The coefficients in chemical equations represent numbers of molecules, not masses of molecules

• Before doing any calculations involving a chemical reaction, be sure the equation for the reaction is balanced

• Calculating Masses of Reactants and Products in Chemical Reaction:

• Balance the equation for th erection

• Convert the known mass of the reactant or product to moles of that substance

• Use the balanced equation to set up the appropriate mole ratios

• Use the appropriate mole ratios to calculate the number of moles of the reactant or product

• Convert from moles back to grams if required by the problem

# 3.10 - Calculations Involving a Limiting Reactant

• Haber process: a very important fertilizer itself and a starting material for other fertilizers made by combining nitrogen

• When chemicals are mixed together to undergo a reaction, they are often mixed in stoichiometric quantities

• The number of products that can form is limited by the methane

• Limiting reactant: The reactant that is consumed first and fore limits the amounts of products that can be formed

• In any stoichiometry calculation involving a chemical reaction, it is essential to determine which reactant is limiting so as to calculate correctly the amounts of products that will be formed

• Theoretical yield: The amount of a product formed when the limiting reactant is completely consumed

• The actual yield of product is often given as a percentage of the theoretical yield, which is called the percent yield