Transcript Chapter 9: The Mole Concept

The Mole Concept
Chapter 9
CHM 130
GCC
9.1 Avogadro’s Number and The Mole
Eggs come in dozens = convenient
12 = dozen
Atoms come in moles
6.02 x 1023 = mole = NA
NA is just a super large number
The Mole
 One mole of softballs would be the size of the Earth!
 But atoms are so tiny that one mole of atoms is
about a handful.
 The mass of a mole of atoms = the atomic mass in
grams in the Periodic Table
1 mole of C atoms is 12.01 grams
1 mole of Na atoms is 22.99 grams
Mole Examples
sugar
water
lead
mercury
copper
sulfur
sodium chloride
9.2 Avogadro’s number :
6.02 x 1023 atoms or mlcls or
1 mol
1 mol
6.02 x 1023atoms or mlcls
1. How many calcium atoms are in 0.250 moles of calcium:
0.250 mol Ca (6.02 x 1023 atoms/mol) = 1.51 x 1023 Ca atoms
2. Calculate the moles of 3.75 x 1027 molecules of oxygen gas
3.75 x 1027 mlcls O2 ( mol/6.02 x 1023 mlcls) = 6230 mol O2
9.3 Molar Mass:
Periodic Table masses are in ( g /mol )
How many grams in one mole of
 carbon?
12.01 g/mol
 gold?
196.97 g/mol
 sodium?
22.99 g/mol
Note = these are all 6.02 x 1023 atoms!
Molar Mass:
Calculate the molar mass of barium hydroxide.
1.What is the formula?
Ba(OH)2
2.Add up the mass for every atom.
137.33 + 2 ( 16.00) + 2 ( 1.01 ) =
171.35 g / mol
Very important – the units are grams per 1 mole, not just grams
9.4 Grams to Moles & Vice Versa
 Use Molar Mass as
(g / mol) or (mol / g)
 How many grams is 1.667 moles of oxygen gas?
 How many moles is 45 grams of silver?
 How many grams is 0.0554 moles of water?
1.667 mol O2 ( 32.00 g/mol) = 53.34 g O2
45 g Ag ( mol/ 107.87 g) = 0.42 mol Ag
0.0554 mol H2O (18.02g /mol) = 0.998 g H2O
9.5 Molar Volume
(V of 1 mole at STP)
Volume of 1 mole of gas
 Gases are mostly empty space so they
ALL have the same volume despite
different mass
 22.4 L per 1 mole of any gas at STP
 STP is Standard Temperature and Pressure
 0°C and 1 atm
For Gases only at STP:
22.4 L / 1 mol or 1 mol / 22.4 L
What is the volume of 1.25 moles of gas
at STP?
1.25 mol ( 22.4 L / 1 mol) = 28.0 L gas
What is the mass of 4.50 L of chlorine gas at STP?
4.50 L Cl2 (mol / 22.4L) (70.90 g/mol) = 14.2 g Cl2
Density
d =
mass
volume
=
molar mass
molar volu me
Calculate the density of ammonia gas, NH3, at STP.
d = (17.04 g/mol ) / ( 22.4 L/mol ) = 0.761 g/L
9.6 Mole Calcs
Example
 How many grams does 17.35 liters of neon gas
mass at STP? How many atoms is this?
 1 mol  20.18g 
17.35 L 

 = 15.6 grams Ne
 22.4L  1 mol 
23
 1 mol  6.02 x 10 atoms
 = 4.66x1023 atoms Ne
17.35 L 

1 mol
 22.4L 

Percent Composition
 = mass % of each element in a compound
 Water is basically 11% H and 89% O
 You divide the total element molar mass by the total
compound molar mass.
 %H = (2.02 g H / 18.02 g H2O) x 100 = 11.2% H
 %O = (16.00 g O / 18.02 g H2O) x 100 = 88.79% O
 Note the %’s should add up to 100% total
Example Problem
 Find the percent composition for TNT, C7H5(NO2)3
Total molar mass is 227.15 grams per mole
% C = (84.07g / 227.15g) x 100 = 37.01% C
% H = (5.05g / 227.15g) x 100 = 2.22%H
% N = (42.03g / 227.15g) x 100 = 18.50%N
% O = (96.00g / 227.15g) x 100 = 42.26%O
Try at home
 How many liters will 3.425 grams of carbon dioxide
gas occupy at STP? 1.74 L
 How many molecules are in 3.425 grams of carbon
dioxide? 4.68 x 1022 molecules
 How many grams will 3.0 x 1015 molecules of carbon
dioxide mass? 2.2 x 10-7 grams
Self Test
 Page 258
 Try 1-7, 10-13
 Answers in Appendix J