1. Molar Mass Conversions and Empirical Formulas

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1. Molar Mass Conversions and Empirical Formulas Powered By Docstoc
					         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
                               The Green Mole
   movie?

b. What did Avogadro teach
   his students in math        Moletiplication
   class?

c. What kind of fruit did      Watermolens
   Avogadro eat in the
   summer?
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          14.01         40.08(2)
+ 78.96(2)     + 1.01(5)     + 55.85
+ 16.00(6)     + 16.00       + 12.01(6)
345.14         35.06         + 14.01(6)
  g/mole         g/mole      + 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              N                O
 207.20 g/mole   28.02 g/mole     96.00 g/mole
 331.22 g/mole   331.22 g/mole    331.22 g/mole

 = 0.6256        = 0.0846         = 0.2898
 or 62.56%       or 8.46%         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:       CH2O
shows the relative       Formaldehyde!
numbers of atoms of      Emp. formula for lots of
each element using the   organic molecules, ex.
smallest whole           C6H12O6
numbers.
                         C3H4O3
Molecular formula:       Does not exist; emp.
tells us actual numbers  formula for ascorbic
of atoms of each         acid: C6H8O6
element in the
molecule.               Molar mass
                         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            0.595 mole O
0.170                    0.170                    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           17.0 g O/16.00 g O

6.376 mole C           6.366 mole H                   1.06 mole O
1.06                   1.06                           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. I will show
    the work on the board.

				
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