Labreport#4 - Determining the Empirical Formula of a Hydrate C PDF

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Determining the Empirical Formula of a Hydrate C...

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General Chemistry SCC 201 Experiment # 3 Determining the Empirical Formula of a Hydrate Prof. Sharmila, Shakya

Objective  

The purpose of this is to determine the percent by mas of water in a Na₂CO₃ Determine the formula of the hydrated form of Na₂CO₃

Introduction For our experiment to design to analyze the empirical formula of a hydrate, we using an empirical formula, which gives the simplest whole number ratio in which the elements are combined. Using a hydrate, an ionic compound that indicates a specific number of water molecules per formula unit of the compound. A compound has H₂O bound to cations. We want to determine the mass of water is hydration lost by heating our experiment. Hydrates can be easily decomposed by mild hating to form water and ionic compounds, called anhydrous. The remaining anhydrous will then be converted to moles of water to construct the empirical formula of the hydrate. Water is part of the crystal structure, removing water form a substance, results in a powdery substance, which means the crystal structure is destroyed. We convert the percentage of each element to the mass of element, convert each element to the moles of each element, and determine the simplest whole number ratio, to analyze how much water percentage in an ionic hydrate was present.

Materials

Chemicals Na₂CO₃ - Sodium Carbonate

Equipment 1x Bunsen burner, 1x ring stand, 1x clay triangle, 1x crucible + cover, 1x electronic scale, 1x tong, 1x wire screen

Method

1. Setting up equipment, using the tong to place crucible white the lid ajar on the clay triangle and supported on the ion ring attached to the ring as shown on Figure 1.

Figure1.

2. Turn on Bunsen burner and place it under the crucible, so that the crucible is hit with a lighter blue flame of the inner cone. Heat for about 5 minutes, than increase heat so that the bottom of the crucible glows red and then heat an additional 3 minutes. 3. Remove crucible from triangle, place it on the wire screen on the lab bench, cover it and allow it to cool down for about 5-10 min. 4. Now weight the cooled crucible without the cover and record the mass on your lab sheet. 5. Use hydrate sample, here, Na₂CO₃ and place 1g into the crucible and weight its mass and record is as mass of crucible + sample on your data sheet. 6. Now place the crucible containing the sample back on the triangle, using the tong to place it and heat it for 5 minutes, using a low flame. (If you can hear a popping, the flame is too high and you need to decrease it). Now heat it for about 10 min but don’t let the bottom of the crucible get red. 7. After 10 min, transfer the crucible on to the wire screen, cover it and allow to cool down for 10 minutes. 8. Weight the crucible without cover, keep sample inside. Record mass of crucible + contents. 9. Obtain the formula for the anhydrous compound.

Mass percent of water %

=

Mass of water lost, g Mass original sample, g

x 100

Moles of compound X, mol = mass of X, g x 1 mol X Molar mass X,g Results Mass of empty crucible

18.7254g

Mass of crucible

19.7582g

Mass after heating and cooling

19.6131g

Sample

Na₂CO₃

Calculation Mass of original ionic hydrate sample

19.7582g – 18.7254g = 1.0328g

Mass of water loss

19.7582g – 19.6131g = 0.1451g H₂O

Mass % of water in the ionic hydrate

0.1451g/1.0328g x 100 = 14%

Mass of anhydrous compound

1.0328 – 0.1451 = 0.8877

Chemical formula of anhydrous compound

Na₂CO₃

Molar mass of anhydrous compound

106g

Number of moles of anhydrous compound

0.8877g/106g = 0.008375472g

Molar mass of water

18g

Number of moles of water loss in sample

0.1454g H₂O x 1 mole/18g H₂O =0.00806111g

Ratio of moles of water/by moles of anhydrous compound

0.00806111g/0.0037375472 = 0.96g ≈ 1

Chemical formula of ionic hydrate

Na₂CO₃ ∙ 1 H₂O

Discussion and conclusion

Our goal in this experience was to determine the percentage of water (by mass), and calculate the ratio of salt to water in a hydrated salt Na₂CO₃. To achieve this known salt was heated and evaporating the water (essentially distillation). We were able to determine the mass loss of water. Using this data, we were able to calculate the mass of original hydrate and the mass percent of anhydrous compound. Doing this we came to the result of our hydrated salt. Also via stoichiometric and molar ratios, we were able to calculate the ratio of salt to water. There were also errors we needed to be aware of when getting accurate numbers. The heating process needed to be carefully pursued. Any water in the evaporating dish ( crucible) accounted as being part of the compound, that’s why we needed to heat up the crucible first, to prevent extra water percentage and would give us a higher mass of water. Also while cooling down, leave cover on the crucible, so no new water molecules could enter the evaporating dish and create new water mass. Possible improvements that could be made to this experiment in the future could include increasing the repeating heating process.

References Miller,D.(2013) General Chemistry I Laboratory Manual.Kendall Hunt....