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Experiment 2 : DETERMINATION OF PERCENT COMPOSITION IN HYDRATE COMPOUNDS
Course Code
: CHM 138
Name
: Muhammad Hafizzudin Bin Norazahar
Matrix No
: 2014837388
Name of Lab Partner : Ahmad Harith Bin Amiruddin (2014206332) Group : AS1161B2 Date of experiment : 6 August 2014
Lectures’s Name : Dr. Rohaiza Binti Saat
Title :
Determination Of Percent Composition In Hydrate Compounds
Objectives :
1) 2)
Procedure : 1.
To calculate the mol of water (x) in barium hydrate (BaCl2.xH2O) . To identify molecular formula of compound A from the thermal decomposition.
A crucible and lid is obtained. The crucible is cleaned and it checked for any stress cracks, fractures or fissures before use. ( If the crucible is dirty, the apparatus is moved to the hood and 1-2mL of 6M HNO3 and it gently evaporated to dryness then inspect the crucible after cooling for any defects. If no defects are found the crucible and lid should supported on a clay triangle ).
2. An initial mass of the crucible and lid is recorded. 3.
The crucible and lid are heated gently for 5 minutes with an intense flame until the bottom of the crucible has become red. The crucible and lid is allowed to cool on the clay triangle before proceed the experiment.
4. The mass of the “ fired ” cool crucible and lid are determined. Then it recorded. Step 2 is repeated until you have two crucible and lid mass readings that differ by no more than 10mg or 0.010g. 5.
Hydrated salt with mass between 1.5g and 2.0g is added to the crucible and the combined mass of the crucible , lid and salt is measured and recorded.
6.
The crucible is placed with the sample on the clay triangle. With the lid slightly jar, the sample is heated slowly for 2 minutes and drastically the sample is heated at high temperature for 10 minutes. The crucible is covered once the heat is removed and cooling is allowed at room temperature in desiccators. The mass of the crucible , lid and anhydrous salt is recorded using the same analytical balance as used in the earlier steps.
7.
The sample is reheated for an additional 2 minutes with intense heat. The combined mass of the crucible , lid and anhydrous salt is weighed and continue repeating this process until two concurrent reading within 10mg of each other were obtained.
8. This procedure is applied for compound A.
Results: Data: a. Determination of Percent Water in Barium Chloride Hydrate ( BaCl2.xH2O)
Mass of empty crucible and lid
38.0857 g
First Heating
38.0838 g
Second Heating
38.0865 g
Mass of crucible + lid + hydrate before heating
39.9602 g
Mass of crucible + lid + hydrate after
38. 3657 g
1) First Heating 2) Second heating
38.3654g
b.
Determination of Compound A
Water Composition and Molecular Formula of
Mass of empty crucible and lid Mass of empty crucible and lid
39.3892 g
First Heating
39.3446 g
Second Heating
39.3466 g
Mass of crucible + lid + hydrate before heating
40.9496 g
Mass of crucible + lid + hydrate after
39.7372 g
1) First Heating 2) Second heating
39.7368 g
Calculations :
a. Determination of Percent Water in Barium Chloride Hydrate (BaCl2.xH2O ) 1. Mass of BaCl2.xH2O. 39.9602 g - 38.0857 g = 1.8745 g
2. Mass of barium chloride anhydrous. 39.9602 g – 38.3657 g = 1.5945 g
3. Mass of water in BaCl2.xH2O. 1.8745 g – 1.5945 g = 0.2800 g
4. Percentage composition of water in BaCl2.xH2O. 0.2800× 100 = 14.9% 1.8745 5. Formula of barium chloride hydrate Number of moles of BaCl2 = Mass/molar mass
Molecular weight of BaCl2 = 208.23 g/mol Number of moles of BaCl2 = 1.5945 g/208.23 g/mol Number of moles of BaCl2 = 0.00766 mol
Number of moles of H2O = Mass/molar mass
Molecular weight of H2O = 18 g/mol Number of moles of H2O = 0.2800/18 g/mol Number of moles of H2O = 0.01556 mol
Ratio: 0.01556 mol/ 0.00766 mol Ratio = 2.03
Therefore, the value of x is equal to 2.
The correct molecular formula is BaCl2.2H2O
Calculations :
b. Determination of Water Composition and Molecular Formula of Compound.
1. Mass of compound A : 40.9496 g - 39.3892 g = 1.5604 g
2. Mass of Compound A anhydrous : 40.9496 g – 39.7372 g = 1.2124 g
3. Mass of water in Compound A 1.5604 g – 1.2124 g = 0.348 g
4. Percentage composition of Compound A 0.348 × 100 % = 22.3 % 1.5604
5. Identification of Compound A ( compare your results to the nearest percent composition of water in hydrated listed ). Li2SO4.H2O, MgSO4.7H2O, FeSO4.7H2O, SrCl2.6H2O, CaSO4.2H2O.
Percentage composition of water of hydrates :
=
( molar mass H2O) x 100% Molar mass of hydrates.
Percentage composition of water in Li2SO4.H2O
=
18 × 100% = 14.06% 2(7) +32 +4(16)+18
Percentage composition of water in MgSO4.7H2O.
=
7(18) × 100% = 51.2% 24 +32 +4(16)+7(18)
Percentage composition of water in SrCl2.6H2O
=
6(18) × 100% = 40.6% 88+2(35)+6(18)
Percentage composition of water in FeSO4.7H2O,
=
7(18) × 100% = 44.8% 59 +32 +4(16)+7(18)
Percentage composition of CaSO4.2H2O :
=
2( molar mass H2O) x 100% Molar mass of CaSO4.2H2O.
=
2(18) 40+32+64+2(18)
=
21 %
x 100%
The percentage of compound A ( 22.3% ) is nearer to the percentage of hydrates ( 21 %) . So, the compound A is CaSO4.2H2O
DISCUSSION: Based on the experiments, we expected to the mass of empty crucible and mass of an empty crucible before and after heating. This is important due to calculate the percentage of composition water in BaCl2.xH2O. Unfortunately, the percentage of composition of water in BaCl2.xH2O (14.9%) is slightly difference to the expected percentage of composition of water in BaCl2.2H2O (14.8%). For compound A , the percentage of compositon of water in Compound A (22.3%) is nearer to the percentage of composition of water in CaSO4.2H2O (21%). So, we identified compound A as CaSO4.2H2O. Make sure we clean the crucible and check for any stress cracks. We also must obey the time given when heating and cooling crucible. When reading the temperature, the eye must be perpendicular to the scale. We also must use the same analytical balance to get more accuracy of mass. Then, repeat the experiment to get average of mass and reading.
Conclusion: Based on the experiment, we found that the mol of water in Barium Chloride Hydrate using present composition concept and the atomic mass.
Question a) Give the reason why the empty crucible should heated before starting the experiment? To make sure no cracks at the crucible and also to remove any moisture present in the crucible by the process of heating which is it absorb any water from crucible. b) Why the process of heating hydrate compound should be started slowly first? To make sure the salt at crucible will not burned and to avoid the water from the salt be dehydrate faster. c) What is the important of percent composition of water in the hydrate compounds? To calculate the mol of water use in the composition.
References: 1) Laboratory Manual Basic Chemistry CHM138 provided by Madam Nurul Huda Abdul Halim, UITM Kuala Pilah. 2) College Matriculation Chemistry SK016, S.A.P Publication Sdn.Bhd 3) Chemistry Reference Book edited by Raymond Chang.
