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					              Determination of Empirical Formula




Prepared by:
Rodney Pujada
Department of Chemistry
Los Angeles Trade Technical College
400 Washington Blvd, Los Angeles CA
April 12th, 2010
                                                                   Rodney Pujada
                                                                   Chemistry 101/ Section 4075
                                                                   Prof: H. Lee
                                                                   Days of class: Tuesday and Thursday
                                                                   Time of class: 6.00 – 9:10 pm
                                                                   Date Due :     12 th,2010



                       Report No 5
Determination of Empirical Formula
                                                                  Name:         Rodney Pujada

                                                             Lab Partner: Armin Vossooghi

 I.        Introduction

The objective of this experiment is to determine the ratio of the element, Mg, in a
compound, based on the law of definite composition: Each compound is characterized
by constant and definite composition. We determine the empirical formula of
magnesium oxide that if formed when it reacts with oxygen gas.

According to this law, the total mass of the products of a chemical reaction must equal
the total mass of the reactants.

                          Mass of Mg + Mass of O2 = Mass of MgxOy

Therefore, the ratio between the number of moles of magnesium used and the number
of moles of oxygen consumed can then be calculated and the empirical formula of
magnesium oxide can be written on the basis of this ration.

 II.       Material

           Crucible and cover, tongs, wire gauze, ring stand,
           iron ring, clay triangle, gas burner, analytical
           balance.



III.       Procedure

3.1 Condition of crucible

   a) Rinse a crucible and its lid with distilled water.
   b) Heat them with a strong flame (two-cone flame with inner blue cone) for 10
       minutes.
   c) Cool down the crucible to room temperature.
   d) Weigh by analytical balance and record the mass of the crucible
3.2 Determine the mass of magnesium.

      a)   Take a strip of the magnesium metal and roll it into a loose ball
      b)   Put it in the crucible and weight together. Record the mass.
      c)   Heat the crucible with the lid completely covering its top.
      d)   Heat 5 minutes with a weak flame.
  e) After 5 minutes, increase to a strong two-cone blue flame.
  f) Use a crucible tong to open the lid slightly, then close it in order for the oxygen
     to come in contact with the metal. Repeat the procedure of opening-closing lid
     several times during the period (about 10 minutes) of heating. (Careful not to
     ignite the magnesium with the flame.)
  g) When the reaction is over, remove the flame and let the crucible cool down.
  h) Open the lid and add a few drops of Water (about 3 or 4 drops) to react with
     magnesium nitride.
  i) Heat the crucible again with the lid slightly open for 2 minutes with gentle
     heating, followed by 10 minutes of very strong heating.
  j) Remove the flame and close the lid, and let the crucible cool down.
  k) Weigh the crucible and contents, and record the mass.


IV.   Data and Calculation


                                                    Run #1     Run #2      Run #3
      Unknown Metal: Magnesium
                                                      (g)        (g)         (g)
      1.      Mass of the empty crucible and lid    46.4167    46.4182     46.4165
      2.    Mass of the crucible, lid, and metal
                                                    46.4984     46.494     46.4962
      (before heating)
      3.    Mass of the crucible, lid, and metal
                                                    46.4993    46.5036     46.5282
      oxide (after heating)
      4.      Mass of the metal                     0.0817      0.0758     0.0797
      5.      Mass of the metal Oxide               0.0826      0.0854     0.1117
      6.      Mass of the oxygen                    0.0009      0.0096      0.032
      7.


                                                    Run #1     Run #2      Run #3
      Data:
                                                      (g)        (g)         (g)
      1.   Mass of the metal oxide                  0.0826     0.0854      0.1117
      2.   Mass of the metal                        0.0817      0.0758      0.0797
      3.   Mass of the oxygen                       0.0009      0.0096      0.032
      Atomic Mass of Magnesium                      24.305      24.305      24.305
      Atomic Mass of Oxygen                         15.994      15.994      15.994
      4.   Moles of metal                          0.0033614   0.003119   0.003279
      5.   Moles of Oxygen                         5.627E-05    0.0006    0.002001
      6.   Ratio                                     59.74       5.20        1.64
      7.   Empirical Formula                        Mg60O       Mg5O        Mg3O2
      True Formula weight of the compound            40.31      40.31       40.31
4.1 Calculate the mass of metal, oxide and oxygen.
    Data:
   1.    Mass of the empty crucible and lid =  m   crucible = 46.4165 g
   2.    Mass of the crucible, lid, and metal (before heating):
        M(metal + crubible) + mcrucible = 46.4962 - 46.4165 = 0.0797 g
   3.    Mass of the crucible, lid, and metal oxide (after heating) :
        M(oxide + crubible) - mcrucible = 46.5282 - 46.4165 = 0.1117 g
   4. Mass of the metal = 0.0797 g
   5. Mass of the metal Oxide = 0.1117 g
   6. Mass of the oxygen = (0.1117 g - 0.0797 g) =           0.032 g

4.2 Calculate the moles of metal and oxygen.
   Data:
   1. Mass of the metal = 0.0797 g
   2. Mass of the metal Oxide = 0.1117 g
   3. Mass of the oxygen = 0.032 g
                                                                      -3
   4. Mole of the magnesium = ( 0.0797 g) / (24.3050 g/mol) = 3.279x10 mol of Mg
                                                                  -3
   5. Mole of the Oxygen = (0.1117 g) / (15.5994 g/mol) = 2.05 x10 mol of O
   6. Ratio moles of Mg / moles of O = 1.5
        Mole of the magnesium …… 1.5 x 2 = 3
        Mole of the Oxygen ………….1.0 x 2 = 2
            We concluded the empirical formula is :    Mg3 O2

   Empirical formula :    Mg3 O2

4.3 Calculate the molecular formula of Magnesium oxide

   Data:
   Calculate n:
   Molecular mass = 40.3 g
   Empirical formula mass = 3 Mg * 24.3050 g = 72.915 g
                               2 O * 15.994 g = 31.988 g
                         Empirical formula mass = 104.903 g

   Replace in the formula.
   N= .       molecular mass .
            Empirical formula mass
   N= .       104.903 g .
            40.3 g
   N= 3
                We concluded the molecular formula of Magnesium oxide (Mg3 O2)3 =
                Mg9 O6 ……….. molecular formula of magnesium oxide.

    V.       Conclusions:

            The molecular mass is 40.3 g. We concluded:

             Mg1 O1 = MgO ==== ( 1 * 24.3050 g of Mg + 1 * 15.994 g of O) =
                                      = 24.3 + 16.0 = 40.3 g of MgO.

            Our empirical formula for magnesium oxide is   Mg3 O2 that is wrong.
            We appreciate the Magnesium does not react completely.

                                  2 Mg        +        O    2            ==   2 MgO
     Experimental mass              0.0797g                  0.032 g            0.1117 g

     Experimental moles        0.003279 mol of Mg     0.002001mol of O.

     Teorical moles:               2 mol of Mg              1 mol of O

        We have 0.003279 mol of Magnesium; therefore, we must have half of 0.003279 g
         for oxygen because the relation mol of Mg and Oxygen is 2 by 1. Finally, we
         needed:
                         (0.003279)/2 =0.0016395 mol of O
        But we had a result 0.002001 mol of O. This error came from because we take in
         count the magnesium react completely.
        Otherwise, Magnesium oxide is produced by the contact with the air, and it is
         possible our magnesium was not chemically pure.
Report No 5 Determination of Empirical Formula



Experiment 6:

Determination of Empirical Formula

This experiment demonstrates the technique used to determine the simple mole ratio
of the elements in a compound. Magnesium metal will be made to react with oxygen of
the air to produce a binary compound, magnesium oxide.

(x) Mg(s) ÷ (y12) O,(g) — MgO(s),

where x and y are the mole ratios of Mg and 0, respectively. The magnesium bums by
heating with a very hot flame. Under high temperatures the magnesium also combines
with the atmospheric nitrogen to produce magnesium nitride, which is not the desired
product for this experiment.

3Mg(s) + N2(g) —÷ Mg3N7(s)

This compound can be converted to the desired product, magnesium oxide, by adding
a few drops of water to the burned reaction mixture, then heating the mixture again.
The excess water is removed by evaporation during the heating step.

Mg3N2(s) + 6 H,O(l) —* 3 Mg(OH)2(s) + 2 NH3(g)

(heat)

? Mg(OH)2(s) —* ? MgO(s) + ? 1-1,0(1)

EQUIPMENT

Crucible and cover, tongs, wire gauze, ring stand, iron ring, clay triangle, gas burner,
balance [Draw a sketch of the apparatus setup].

PROCEDURE

1. Clean a porcelain crucible and its lid. -

1.1. Rinse them with distilled water.

1.2. Heat them with a strong flame (two-cone flame with inner blue cone) for 8 to 10
minutes.

1.3. Turn off the flame and let the crucible cool down to room temperature.

2. Weigh the crucible and the lid carefully using an analytical balance, and record the
inass.

3. Determine the mass of magnesium.
3.1. Obtain a strip of the magnesium metal (approximately 7 to 10 cm long).

3.2. Roll the magnesium strip into a loose ball

3.3. Place it in the crucible.

3.4. Weigh and record the mass of the crucible, lid and Mg, using the same analytical
balance.

4. Heat the cnicible with the lid completely covering its top.

4.1. The heating must be gentle at first (about 5 minutes) with a weak flame.

4.2. After 5 minutes, increase to a strong two-cone blue flame.

5. Use a crucible tong to open the lid slightly, then close it in order for the oxygen to
come in contact with the metal. Repeat the procedure of opening-closing lid several
times during the period (about 10 minutes) of heating. (Careful not to ignite the
magnesium with the flame.)

6, When the reaction is over, remove the Ilame and let the crucible cool down.

7. Open the lid and add a few drops of Water (about 3 or 4 drops) to react with
magnesium nitride.

8. Heat the crucible again with the lid slightly open for 2 minutes with gentle heating,
followed by 10 minutes of very strong heating.

9. Remove the flame and close the lid, and let the crucible cool down.

10. Weigh the crucible and contents, and record the mass.

RESULTS _____ _____

1. Unknown number (or the metal name) ____________ ______________

2. Mass of the crucible, lid, and metal (before heating) ____________ ______________

3. Mass of the crucible, lid, and metal oxide (after heating) ____________ ______________

4. Mass of the empty crucible and lid ____________ ______________

5 Atomic mass of the metal ______________________

6. Atomic mass of the oxygen atom ____________________

7. Mass of the metal oxide ________ ________

8. Mass of the metal _______ __________

9. Mass of the oxygen ________— ________

10. Moles of metal ___________ —___________
11. Moles of oxygen ____________ _____________

12. Divide your answers in (10) and (11) with the smallest answer, and report the mole
ratio as

1 mole of___________ ________ mole of

13. What is the empirical formula of the compound? ____________________

14. Ask your instructor for the true formula weight of the compound, then determine
the formula of this metal oxide.

CONCLUSIONS

Run #1 Run #2

CALCULATIONS

Run #1

Run #2

				
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Description: The objective of this experiment is to determine the ratio of the element, Mg, in a compound, based on the law of definite composition: Each compound is characterized by constant and definite composition. We determine the empirical formula of magnesium oxide that if formed when it reacts with oxygen gas.