Determination of Empirical Formula

Prepared by:Rodney PujadaDepartment of ChemistryLos Angeles Trade Technical College400 Washington Blvd, Los Angeles CAApril 12th, 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 ballb) Put it in the crucible and weight together. Record the mass.c) Heat the crucible with the lid completely covering its top.

Rodney PujadaChemistry 101/ Section 4075Prof: H. LeeDays of class: Tuesday and Thursday

Time of class: 6.00 – 9:10 pmDate Due : 12 th,2010

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

Unknown Metal: MagnesiumRun #1

(g)Run #2

(g)Run #3

(g)1.        Mass of the empty crucible and lid 46.4167 46.4182 46.41652.        Mass of the crucible, lid, and metal (before heating) 46.4984 46.494 46.4962

3.        Mass of the crucible, lid, and metal oxide (after heating) 46.4993 46.5036 46.5282

4.        Mass of the metal 0.0817 0.0758 0.07975.        Mass of the metal Oxide 0.0826 0.0854 0.11176.        Mass of the oxygen 0.0009 0.0096 0.0327.                

 Data:Run #1

(g)Run #2

(g)Run #3

(g)1.       Mass of the metal oxide 0.0826 0.0854 0.11172.       Mass of the metal 0.0817 0.0758 0.07973.       Mass of the oxygen 0.0009 0.0096 0.032Atomic Mass of Magnesium 24.305 24.305 24.305Atomic Mass of Oxygen 15.994 15.994 15.994

4.      Moles of metal 0.0033614 0.0031190.00327

9

5.      Moles of Oxygen 5.627E-05 0.00060.00200

16.       Ratio 59.74 5.20 1.647.       Empirical Formula Mg60O Mg5O Mg3O2

True Formula weight of the compound 40.31 40.31 40.31

IV.1 Calculate the mass of metal, oxide and oxygen.Data:

1. Mass of the empty crucible and lid = mcrucible = 46.4165 g2. 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 g5. Mass of the metal Oxide = 0.1117 g6. Mass of the oxygen = (0.1117 g - 0.0797 g) = 0.032 g

IV.2 Calculate the moles of metal and oxygen.Data:1. Mass of the metal = 0.0797 g2. Mass of the metal Oxide = 0.1117 g3. Mass of the oxygen = 0.032 g

4. Mole of the magnesium = ( 0.0797 g) / (24.3050 g/mol) = 3.279x10-3

mol of Mg

5. Mole of the Oxygen = (0.1117 g) / (15.5994 g/mol) = 2.05 x10-3

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

IV.3 Calculate the molecular formula of Magnesium oxide

Data: Calculate n:Molecular mass = 40.3 gEmpirical formula mass = 3 Mg * 24.3050 g = 72.915 g

2 O * 15.994 g = 31.988 gEmpirical formula mass = 104.903 g

Replace in the formula.N= . molecular mass .

Empirical formula massN= . 104.903 g .

40.3 gN= 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

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CALCULATIONS

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