Warmup: 5 minutes
1. Why do we use moles?
Makes it easier to measure chemical quantities
because atoms and molecules are very small.
2. Define the following ‘molecabulary’ words
demoleition
molearchy
molectomy
3.
a. What is a mole's favorite movie?
b. What did Avogadro teach his students in math
class?
c. What kind of fruit did Avogadro eat in the summer?
Molar Mass Conversions and
Empirical Formulas
*need calculator and PT
demoleition - the destruction which moles bring about in
your yard.
molearchy - government in which moles are in complete
control; under this government Mole Day is celebrated
twice a year and chemistry is the only scientific subject
taught in school
molectomy - the process of removing a mole's insides
a. What is a mole's favorite
movie?
The Green Mole
b. What did Avogadro teach
his students in math
class?
Moletiplication
c. What kind of fruit did
Avogadro eat in the
summer?
Watermolens
1: What is the molar mass of Na2SO4?
Na: 2 mole x 22.99 = 45.98
S: 1 mole x 32.07 = 32.07
O: 4 mole x 16.00 = 64.00
= 142.05 g/mole
Calculating Molar Mass
1) Find the atomic mass of each element
2) Multiply the number of moles times the
atomic mass of for each element
3) Add values, round to the hundredths place,
and report your answer in g/mole
2. Calculate the molar masses of the following
compounds:
a. Zr(SeO3)2
b. NH4OH c. Ca2Fe(CN)6.12H20
91.22
+ 78.96(2)
+ 16.00(6)
345.14
g/mole
14.01
+ 1.01(5)
+ 16.00
35.06
g/mole
40.08(2)
+ 55.85
+ 12.01(6)
+ 14.01(6)
+ 1.01(24)
+ 16.00(12)
508.37 g/mole
Ex 3: What is the molar mass of Pb(NO3)2?
Pb: 1 mole x 207.20 = 207.20
N: 2 mole x 14.01 = 28.02
O: 6 mole
x 16.00 = 96.00
= 331.22 g/mole
Ex 4: Find the mass % of each element in Pb(NO3)2
Pb
207.20 g/mole
N
28.02 g/mole
331.22 g/mole
331.22 g/mole
O
96.00 g/mole
331.22 g/mole
= 0.6256
or 62.56%
= 0.0846
or 8.46%
= 0.2898
or 28.98%
5. What are the mass percents of iron and
oxygen in Fe2O3?
Iron: (2 x 55.85 g/mole) x 100= 69.94 %
(159.7 g/mole)
Oxygen: (3 x 16.00 g/mole) x 100 = 30.06 %
(159.7 g/mole)
Conversions using Molar Mass
Ex 6: How many moles is 5.69 grams of CH4 ?
5.69 g CH4 ( 1 mole CH4) = 0.355 mole CH4
( 16.05 g CH4)
Ex 7: How many moles is 4.56 g of CO2 ?
4.56 g CO2 ( 1 mole CO2) = 0.104 mole CO2
( 44.01 g CO2)
You need the molar mass of carbon dioxide
Ex 8: Find the mass (in grams) of 9.8 moles of
lithium fluoride
9.8 mole LiF ( 25.94 g LiF) = 250 g LiF
( 1 mole LiF)
You need the formula and molar mass of this
compound FIRST!
Empirical formula:
shows the relative
numbers of atoms of
each element using the
smallest whole
numbers.
CH2O
Formaldehyde!
Emp. formula for lots of
organic molecules, ex.
C6H12O6
C3H4O3
Does not exist; emp.
formula for ascorbic
acid: C6H8O6
Molecular formula:
tells us actual numbers
of atoms of each
element in the
Molar mass
molecule.
must be known.
They CAN BE the same.
9. The empirical formula of styrene is CH; its formula
weight (‘molar mass’) is 104.1 g/mole. What is the
molecular formula of styrene?
molecular formula = multiple of the empirical formula
* molar mass of the compound is needed
Molar mass styrene = 104.1 g/mole
Molar mass emp.form. = 13.02 g/mole
= 7.995391… = 8
molecular formula = C8H8
10. A 25.0 gram sample of a compound contains 6.64 grams
potassium, 8.84 g chromium, 9.52g oxygen. Find the empirical
formula of this compound.
Step 1: Convert each mass to moles
0.170 mole K
0.170 mole Cr
0.170
0.170
0.595 mole O
0.170
Step 2. Get ratio of moles (by dividing each # moles by smallest
# moles)
Mole ratio: 1 mole K: 1 mole Cr: 3.50 moles O
Step 3: If the numbers in the ratio are not integers, double, triple,
etc each number until they are all integers….you may want to
round slightly (ex. 2.98 can be 3 but 2.49 should be doubled to
4.98, then rounded to 5)
New ratio: 2 : 2 : 7
K2Cr2O7, potassium dichromate
11. Phenol is a compound which contains 76.57% carbon, 6.43%
hydrogen, and 17.0% oxygen. What is the empirical formula of phenol?
If you are given percents of elements instead of actual masses, turn
each percent into a gram amount out of a 100g sample
Step 1: Convert each mass to moles
76.57g C/12.01 g C 6.43 g H/1.01g H
6.376 mole C
1.06
6.366 mole H
1.06
17.0 g O/16.00 g O
1.06 mole O
1.06
Step 2. Get ratio of moles (by dividing each # moles by smallest #
moles)
Mole ratio: 6 mole C: 6 mole H: 1 moles O
C6H6O = phenol!
12. A 5.00 gram sample of an acid contains 2.00 g
carbon, 0.336 g hydrogen, and 2.66 g oxygen. Find
the molecular formula of this acid and give the proper
name if its molar mass = 60.06 g/mole
Try this yourself. It is acetic acid. We will do the work
on the board in a bit for 12 and 13.
13. Monosodium glutamate (MSG), a food flavor enhancer,
has been blamed for "Chinese restaurant syndrome", the
symptoms of which are headaches and chest pains
MSG has the following composition by mass:
35.51% carbon 4.77 % hydrogen 37.85 % oxygen
8.29 % nitrogen
13.60 % sodium
What is its molecular formula it its molar mass is 169 g?