Determination of Percentage Composition in Hydrate Compounds

Objectives:

1. Calculate the moles of water (x) in barium hydrate (BaCl2.xH2O).
2. Identify the molecular formula of compound A through thermal decomposition.

Procedure:

1. Obtain a crucible and lid, ensuring they are clean and free of defects. If dirty, treat with 6M HNO3, evaporate to dryness, and inspect after cooling.
2. Record the initial mass of the crucible and lid.
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3. Heat the crucible and lid gently for 5 minutes until the bottom turns red. Cool on a clay triangle.
4. Determine the mass of the cooled, fired crucible and lid. Repeat until two readings differ by no more than 10mg.
5. Add hydrated salt (1.5g to 2.0g) to the crucible, record the combined mass of crucible, lid, and salt.
6. Heat the sample on the clay triangle with the lid slightly ajar for 2 minutes, then intensely for 10 minutes. Cover the crucible and cool in a desiccator. Record the mass of the crucible, lid, and anhydrous salt.
7. Repeat the heating process until two consecutive readings within 10mg are obtained.
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8. Apply the same procedure for compound A.

This laboratory aims to determine the percentage composition in hydrate compounds by calculating the moles of water and identifying molecular formulas through thermal decomposition. Crucibles are carefully prepared, heated, and weighed at various stages to obtain precise measurements. The procedure is meticulously executed for both barium hydrate and compound A to ensure accurate results in understanding their composition.

Results

Determining the Percentage of Water in Barium Chloride Hydrate (BaCl2.xH2O):

1. Find the mass of BaCl2.xH2O: 39.9602 g - 38.0857 g = 1.8745 g
2. Calculate the mass of anhydrous barium chloride: 39.9602 g – 38.3657 g = 1.5945 g
3. Determine the mass of water in BaCl2.xH2O: 1.8745 g – 1.5945 g = 0.2800 g
4. Calculate the percentage of water in BaCl2.xH2O: (0.2800 g / 1.8745 g) × 100 = 14.9%
5. Find the number of moles of BaCl2 and H2O: a. Moles of BaCl2 = 1.5945 g / 208.23 g/mol = 0.00766 mol b. Moles of H2O = 0.2800 g / 18 g/mol = 0.01556 mol c. Ratio of moles: 0.01556 mol / 0.00766 mol = 2.03
6. Therefore, x is equal to 2, and the correct molecular formula is BaCl2.2H2O.

b. Determining Water Composition and Molecular Formula of Compound A:

1. Calculate the mass of compound A: 40.9496 g - 39.3892 g = 1.5604 g
2. Find the mass of anhydrous compound A: 40.9496 g – 39.7372 g = 1.2124 g
3. Determine the mass of water in compound A: 1.5604 g – 1.2124 g = 0.348 g
4. Calculate the percentage composition of compound A: (0.348 g / 1.5604 g) × 100 = 22.3%
5. Compare the result to known hydrates: a. Li2SO4.H2O: 14.06% b. MgSO4.7H2O: 51.2% c. SrCl2.6H2O: 40.6% d. FeSO4.7H2O: 44.8% e. CaSO4.2H2O: 21%
6. Compound A's percentage (22.3%) is closest to CaSO4.2H2O (21%), so the compound is CaSO4.2H2O.

In our experiments, we anticipated determining the mass of the empty crucible and the mass of the crucible both before and after heating. This step is crucial for calculating the percentage composition of water in BaCl2.xH2O. Unfortunately, the observed percentage composition of water in BaCl2.xH2O (14.9%) slightly deviates from the expected percentage composition for BaCl2.2H2O (14.8%). Conversely, for compound A, the observed percentage composition of water (22.3%) closely aligns with the expected percentage composition of water in CaSO4.2H2O (21%). Consequently, we identified compound A as CaSO4.2H2O.

To ensure accuracy in our experiments, we meticulously cleaned the crucible, checking for any stress cracks. It was imperative to adhere to specified heating and cooling durations. Additionally, when reading temperatures, we ensured that the eye was perpendicular to the scale. Maintaining consistency, we used the same analytical balance to enhance mass accuracy. Subsequently, we repeated the experiment to obtain an average mass and reading.

In conclusion, our experiment determined the mole of water in Barium Chloride Hydrate by applying the concept of present composition and atomic mass.