Chapter 10

Measuring MatterMatter can be measured in three ways

By massBy volumeBy counting

Measuring by countingThe mass of a penny is 2.500 gYou collect pennies in an empty 2-L bottle

with a mass of 77.00 g. After a few months, the mass of your collection is 2.297 kg. How many pennies do you have?

The mass of a nickel is 5.000 gIf you have 625 g of nickels, how many

pennies do you need to equal the mass of nickels?

Measuring by countingThe mass of a penny is 2.500 gThe mass of a nickel is 5.000 gWhat is the ratio of the masses of the coins?If you have 625 g of nickels, how many

pennies do you need to equal the mass of nickels?

The MoleA dozen is a counting unit

How many eggs are in 12 dozen?42 eggs is how many dozen eggs?

The mole is the SI unit for quantity of a substanceIt is a counting unitOne mole contains 6.02 x 1023 represetative

particles1 mol O2 = 6.02 x 1023 O2 molecules1 mol Na+ ions = 6.02 x 1023 Na+ ions How many atoms are in 1.5 mol He?

Mass and MolesThe atomic mass of an element in amu’s =

the mass of one mole of the element in grams.The atomic mass of Ne is 20.18 amuThe molar mass of Ne is 20.18 gWhat is the molar mass of Ca?What is the molar mass of N2?

Molar Mass of a CompoundMolar mass is the mass of one mole of a

substance.Molar mass of a compound is the total sum of

the molar masses of the elements in the compound

The molar mass of H2O is…Element amu x # atomsH 1.01 x 2 = 2.02O 16.00x 1 16.00Sum 18.02 g/mol

Mole-Mass RelationshipsMolar mass is the mass of one mole of a

substanceMolar mass is used to convert between moles

and massFind the mass of 3.00 mol of NaCl

Find the number of moles in 86.68 g of NaCl

NaCl g 175.35NaCl mol

NaCl g 58.45NaCl mol 3.00

NaCl mol 1.48NaCl g 58.45

NaCl 1molNaCl g 86.68

Mole-Volume RelationshipsAvogadro’s hypothesis: volume of a gas is

proportional to the moles of a gasV α n Standard temperture and pressureT = 00C = 273.15K; P = 1 atmAt STP, one mole of any gas occupies 22.4 LHow many moles O2 are in 33.6 L of the gas

at STP?Find the volume of 3.5 mol of N2 at STP.

Molar Mass & Density of GasesRemember that density is the ratio of mass to

volume

Gases are not very dense, so the units are grams/liter

The molar mass of a gas can be determined using the standard molar volume of gas and the density of the gas

V

md

mol

g

mole

L 22.4

L

g

MassMolar VolumeMolar Density

Molar Mass & Density of GasesA gas containing carbon and oxygen has a

density of 1.964 g/L at STP. Find its molar mass.

Find the density of krypton gas at STP.

Find the density of a gas with a molar mass of 80 g/mol.

Mole Road Map

Percent CompositionThe law of definite proportions states a substance will

always have the same proportions of elements by mass

This means that each substance has a known percent composition by mass

% comp is the mass of an element within a compound divided by the molar mass of the compound

Example:%N in NH3 = mass of N/molar mass of NH3%N = 14.01 g/17.04 g =0.822 = 82.2%%H = 1-0.882 = 0.178 = 17.8%

Percent Compositon PracticeDetermine % composition of glucose, C6H12O6

Find the % composition of potassium dichromate, K2Cr2O7

Empirical FormulasEmpirical formulas show the smallest whole

number ratio of elements in compound.Examples:

CH CH4 CH2O HOSO3

In all these examples, the ratios of elements cannot be reduced

Determining Empirical FormulaEmpirical formulas can be determined from

percent composition dataPercent composition can be used as a

conversion factorExample: a compound is 25.9% N and 74.1%

O. Determine its empirical formula

Determining Empirical Formula from Percent CompositionConvert % composition to moles

N1.85 O4.63 ?????

Element

% com

p

Mass in 100

g

Moles in 100 g

N 25.9 25.9 1.85

O 74.1 74.1 4.63

Determinig Empirical Formula from Percent Composition DataHow do you convert the subscripts to small

whole numbers?Begin by dividing by the smallest number of

moles

2.51

1.85

4.63

1.85

1.85 ONON

Determining Empirical Formula from Percent Composition DataIf the result is still not whole numbers,

multiply the subscripts by a number that will give you whole number mole ratios

52

22.521

ON

ON

Molecular FormulasMolecular formulas are some whole number

multiple of empirical formulas.

Determining Molecular Formulas from Empirical FormulasMolecular formula = n x Empirical formulaUsually you will be given the molar mass of

the compoundCalculate the empirical formula massDivide molar mass by efm to determine nThen multiply the empirical formula by n to

get the molecular formula

Determining Molecular Formula from Empirical FormulaGiven molar mass = 60.0 g/mol and empirical

formula equals CH4N, find the molecular formula.

Mol mass = n(empirical formula mass)

282

4

NHC

N)2(CH

formula empirical2 formula molec

230.0g

60.0g

efm

mass mol

n