Chemical Composition

8.1 Counting by WeighingWhich method is more effective?

Counting jelly beans vs. weighing jelly beantotal mass

Average mass = ------------------number of samples

8.2 Atomic Mass: Counting Atoms by WeighingAtomic mass unit (amu): unit of mass for an

atom1 amu = 1.66 x 10-24 g

ExampleCalculate the mass in amu of a sample of

aluminum contains 75 atomsCarbon contains 62 atoms

Calculate the number of sodium atoms present in a sample that has

1172.49 amuCopper atoms present in a sample that has a

mass of 1779.4 amu

8.3 The MoleSI unit for an amount of substance

Countable unitsCan be obtained from a balanced equationE.g C(s) + O2(g) CO2(g)

1 atom of C reacts with 1 molecule O2 to yield 1 molecule of CO2

OR1 mol of C reacts with 1 mole O2 to yield 1 mole

CO2

8.3 The MoleDescribed the number of objects present in a dozen

Based on C-12

1 mol of anything = 6.02 x 1023 units of that substance (atoms, molecules, particles, dollars etc..)Avogadro’s number

1 mol of an atom = atomic weight of an atom in gramsUse as a conversion factorE.g Atomic weight of Na = 22.99 amu

1 mol Na = 22.99 g Na

Steps in determination the moleDetermine the final unitDetermine the atomic mass of an atomWrite out an equivalenceExpress equivalence as a conversion factor

ExampleCalculate the number moles of atoms and the

number of atoms in 2.50 g sample of calcium57.7 g sample of sulfur

8.4 Molar MassLike density, it’s a ratio of two numbers.

Mass per one mole of substanceUnit => g/mol

E.g if 0.400 mol of a compound has a mass of 17.6 g, what is the molar mass of the compound?

Replaced with molecular weight because molar mass described the concept more accurately

Formula weight is used for ionic compound

ExampleWhat is the molar mass of CH4?Calculate molar mass of sulfur dioxide, a gas

produced when sulfur containing fuels are burned.

Calculate the molar mass for each of the substance below (record to two decimal places)WaterAmmoniaBarium hydroxide

Calculating the Mass from MolesRecall

Molar mass is the mass per one mole of a substance

1 mol of substance = # gUse as a conversion factor

E.g if the molar mass of NaCl = 58.44 g/mol

ExampleCalcium carbonate (also called as calcite), is

the principal mineral found in limestone, marble, chalk, pearls and the shells of marine animals such as clamsCalculate the molar mass of calcium carbonateA certain sample of calcium carbonate contains

4.86 mol. What is the mass in grams of this sample?

ExampleCalculate the mass of 4.85 mol of acetic acid,

HC2H3O2. Vinegar is a dilute solution of acetic acid

Calculate the molar mass of sodium sulfate. A sample of sodium sulfate with a mass of 300.0 g represents what number mole of sodium sulfate?

How many water molecules are in a 10.0g sample of water?

8.5 Percent CompositionPercent composition – the mass present of each

element present in a compound

% composition =

To calculate the percent composition (percentage composition) of a compound Calculate the molar mass of the compound Calculate the total mass of each element present in the

formula of the compound Calculate the percent composition (percentage

composition):% by weight (mass) of element = (total mass of element present ÷ molecular mass) x 100

ExampleCarvone 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. Calculate the mass of percent of each element in carvone.

Determine the mass percent of each element in sulfuric acid.

8.6 Formula of CompoundsFormula of a compound represents the

relative numbers of various types of atoms present.E.g CO2, H2O etc..

Empirical formula: simplest formulaMolecular formula: the actual formula of a

compound

8.7 Calculation of Empirical FormulaIn each case below, the molecular formula for a

compound is given. Determine the empirical formula for each compoundC6H6. This is the molecular for benzene; a liquid

commonly used in industry as a starting material for many important products

H2O2. This is hydrogen peroxide, a substance commonly diluted with water and used as a disinfectant

CCl4. This is carbon tetrachloride, an organic solvent

8.7 Calculation of Empirical FormulaStep 1: Obtain the mass of each element present (in

grams)Step 2: Determine the number of moles of each type of

atom presentStep 3: Divide the number of mole of each element by

the smallest number of moles to convert the smallest number to 1. If all of the numbers so obtained are integers, these are the subscripts in the empirical formula. If one or more of these number are not integers, go on to step 4

Step 4: Multiply the numbers you derived in step 3 by the smallest integer that will convert all of them to whole numbers. This set of whole numbers represents the subscripts in the empirical formula

ExampleIn a lab experiment it was observed that 0.6884 g

of lead combines with 0.2356 g of chlorine to form a binary compound. Calculate the empirical formula of this compound

When a 2.00 g sample of iron metal is heated in air, it reacts with oxygen to achieve a final mass of 2.573 g. Determine the empirical formula for this iron oxide

Calculating Empirical Formula from Percent CompositionCisplatin, the common name for a platinum

compound that is used to treat cancerous tumors, has the composition (mass percent) 65.02% platinum, 9.34 % nitrogen, 2.02% hydrogen, and 23.63% chlorine. Calculate the empirical formula for csiplatin

8.8 Calculation of Molecular FormulasMolecular formula – gives the actual

numbers of atoms in a moleculeMolecular formula = n x empirical formula

may be the same as the empirical formula

a multiple of the empirical formula (or # of empirical unit) is defined:

Multiple ( # of empirical unit) or n =

ExampleA compound used as an additive for

gasoline to help prevent engine knock shows the following percent composition

71.65 % Cl 24.27 % C4.07 % H

The molar mass is known to be 98.96 g/mol. Determine the empirical formula and the molecular formula for this compound

A compound containing carbon, hydrogen, and oxygen is found to be 40.00% C and 6.700% by mass. The molar mass of this compound is between 115 g/mol and 125 g/mol. Determine the molecular formula for this compound