Mole Calculations
Lesson 3
Avogadro’s number
Review
The mole is the fundamental unit in chemistry for measuring the AMOUNT OF
SUBSTANCE. We can convert from moles to number of particles.
1 mole = 6.02 x 1023 particles or molecules or atoms!
CONVERSION FACTOR
1 mol
6.02 x 1023
OR
6.02 x 1023
1 mol
•
This is Avogadro's number
•
There are no set units for Avogadro’s number because it is simply a number. You
can use any of the units(particles, molecules or atoms) depending on the question.
Molar mass from Periodic
Table
Molar Mass Units
# grams
1 mole
or
1 mole
# grams
The molar mass is a new conversion factor that can convert between
grams and moles.
Avogadro’s number is another conversion factor that can convert
between moles and particles
Particles
1.00 mole =
6.02 x 1023 particles
Moles
Grams
Molar Mass
From Periodic Table
g / mole
Try
Ex: How many moles of NaBr are there in 1.7 x 1025 molecules of
NaBr?
Ex: How many molecules of LiCl are there in 0.5 moles of LiCl?
Try
Ex: how many grams are in 3 moles of Al2S3
STP
Recall Avogadro’s hypothesis:
– Equal volumes of different gases contain the same number of
particles (at the same temperature and pressure).
Standard Temperature and Pressure
Chemists have a set STANDARD for temperature and pressure:
– STP = standard temperature and pressure
– STP = 0 ºC and 101.3 kPa
Molar Volume
Just like we have a molar mass, we also have a molar volume.
•
The molar volume of a gas is the VOLUME occupied by ONE MOLE of the
gas.
• It is a fact that at STP, 1 mol of any gas occupies 22.4 L of volume
– this is known as ‘molar volume’
• We then get another CONVERSION FACTOR
1 mol
OR
22.4L
22.4 L
1 mol
BUT ONLY AT STP, and only for gases!
The Molar Volume
Ex: How many L will 2.3 mol of He (g) occupy at STP?
Ex: How many mol of Ne (g) will fill a 400L container at STP?
Find # of atoms in a molecule
How many atoms are there in 1 molecule of CuSO4 · 5H2O?
How many atoms are there in 15 molecules of CuSO4 · 5H2O?
Always find the # of atoms in 1 molecule first, then multiply by # of molecules (15)
How many H atoms are in CuSO4 · 5H2O?
CONVERSION
CONVERSION FACTOR
Moles
↔
# of Particles
(Avogadro’s number)
Moles
↔
Mass
(Molar mass)
Moles
↔
Volume
(gas @ STP)
(Molar Volume)
Molecules ↔ Atoms
The MOLE is CENTRAL to all conversions between
mass, volume and particles.
MULTIPLE CONVERSIONS
Ex: Find the mass, (in grams), of 1.25L of NH3 (g) at STP:
Ex: What is the volume occupied by 45.0g of KOH(g) at STP?
MULTIPLE CONVERSIONS
Ex: What is the mass of 175 N (Nitrogen) atoms?
MULTIPLE CONVERSIONS
What if you want to find the volume of a solid or a
liquid????
Use densityd = m/V
If the volume of a liquid or solid is unknown....use V = m/d (you can always
find the mass from the moles of substance present)
This is because….you cannot use 22.4L when
calculating a volume of liquid or solid (only
gases!)
MULTIPLE CONVERSIONS
Ex: What is the volume occupied by 5.00 mol of ethanol, CH
(the density of ethanol is 0.790 g/mL)
3
CH2 OH(l)?
MULTIPLE CONVERSIONS
• If the number of moles is unknown...
• use the density and volume to calculate mass
• m = d x V and
• then convert mass to moles
MULTIPLE CONVERSIONS
Ex: How many moles of Hg (l) are contained in 56 mL of Hg(l)? (d =
13.6 g/mL)
MULTIPLE CONVERSIONS
Ex: CCl4 (l) has a density of 1.59 g/mL. How many atoms of C are in
200 mL of CCl4?
MULTIPLE CONVERSIONS
Ex: What is the density of CH2F2 (g) at STP?
MULTIPLE CONVERSIONS
If the molar mass of a gas at STP is
unknown ( in other words, you don’t have the
formula for the gas)
Find the density of the gas and then
combine the density with volume of 1 mol
(22.4L) to find the mass of 1 mole
MULTIPLE CONVERSIONS
Ex: A 3.0 L bulb contains 2.2 g of a gas at STP. What is
the molar mass of the gas?
Helpful hints
- Write out what is given in the question with its units(on left)
- Write out what units your answer has to end up with(on right)
- Using the different conversion factors you know, start
multiplying to cancel out the units you don’t want, and
keep the ones you want.
Homework
Page 88, 89, 90
#35, #36(b,d), #37 (d), #38(b), #39(d,f)
#40(a,f) #41(a,d,f, j) #42(a,c,e,i,l,s)
#43(a,d,g,i,k,o,g,r)
You must practice these questions, or you
will be lost for this unit ….