Transcript molar mass of the compound

Chapter 10
Measuring Matter
 Matter can be measured in three ways
 By mass
 By volume
 By counting
Measuring by counting
 The mass of a penny is 2.500 g
 You 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 g
 If you have 625 g of nickels, how many pennies do you
need to equal the mass of nickels?
Measuring by counting
 The mass of a penny is 2.500 g
 The mass of a nickel is 5.000 g
 What 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 Mole
 A 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 substance
 It is a counting unit
 One mole contains 6.02 x 1023 represetative particles
 1 mol O2 = 6.02 x 1023 O2 molecules
 1 mol Na+ ions = 6.02 x 1023 Na+ ions
 How many atoms are in 1.5 mol He?
Mass and Moles
 The 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 amu
 The molar mass of Ne is 20.18 g
 What is the molar mass of Ca?
 What is the molar mass of N2?
Molar Mass of a Compound
 Molar 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
 H
 O
 Sum
amu x
1.01
x
16.00 x
# atoms
2
=
1
2.02
16.00
18.02 g/mol
Mole-Mass Relationships
 Molar mass is the mass of one mole of a substance
 Molar mass is used to convert between moles and mass
 Find the mass of 3.00 mol of NaCl
58.45 g NaCl
3.00 mol NaCl 
 175.35 g NaCl
mol NaCl
 Find the number of moles in 86.68 g of NaCl
1mol NaCl
86.68 g NaCl 
 1.48 mol NaCl
58.45 g NaCl
Mole-Volume Relationships
 Avogadro’s hypothesis: volume of a gas is proportional
to the moles of a gas
 Vαn
 Standard temperture and pressure
 T = 00C = 273.15K; P = 1 atm
 At STP, one mole of any gas occupies 22.4 L
 How 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 Gases
 Remember that density is the ratio of mass to volume
m
d
V
 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
Density  Molar Volume  Molar Mass
g 22.4 L
g


L mole
mol
Molar Mass & Density of Gases
 A 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 Composition
 The 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 Practice
 Determine % composition of glucose, C6H12O6
 Find the % composition of potassium dichromate,
K2Cr2O7
Empirical Formulas
 Empirical formulas show the smallest whole number
ratio of elements in compound.
 Examples:
CH
CH4
CH2O
HO
SO3
 In all these examples, the ratios of elements cannot be
reduced
Determining Empirical Formula
 Empirical formulas can be determined from percent
composition data
 Percent composition can be used as a conversion factor
 Example: a compound is 25.9% N and 74.1% O.
Determine its empirical formula
Determining Empirical Formula
from Percent Composition
 Convert % composition to moles
Element
%
Mass in
comp
100 g
Moles in
100 g
N
25.9
25.9
1.85
O
74.1
74.1
4.63
 N1.85 O4.63 ?????
Determinig Empirical Formula from
Percent Composition Data
 How do you convert the subscripts to small whole
numbers?
 Begin by dividing by the smallest number of moles
N 1.85 O 4.63  N1O 2.5
1.85
1.85
Determining Empirical Formula
from Percent Composition Data
 If the result is still not whole numbers, multiply the
subscripts by a number that will give you whole
number mole ratios
N 12 O 2.52 
N 2O5
Molecular Formulas
 Molecular formulas are some whole number multiple
of empirical formulas.
Determining Molecular Formulas
from Empirical Formulas
 Molecular formula = n x Empirical formula
 Usually you will be given the molar mass of the
compound
 Calculate the empirical formula mass
 Divide molar mass by efm to determine n
 Then multiply the empirical formula by n to get the
molecular formula
Determining Molecular Formula
from Empirical Formula
 Given molar mass = 60.0 g/mol and empirical formula
equals CH4N, find the molecular formula.
 Mol mass = n(empirical formula mass)
mol mass 60.0g
n

2
efm
30.0g
molec formula  2empirical formula
 2(CH 4 N)
 C2 H8 N 2
