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Lab # 2 The Formula Of Hydrates

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Hydrates Hydrates are ionic compounds that have a specific number of water molecules attached to them. Examples: Calcium Chloride dihydrate – CaCl 2 * 2 H 2 O Copper (II) Sulfate pentahydrate – CuSO 4 * 5 H 2 O

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Analyzing a Hydrate When a hydrate is heated the water molecules that are attached to it evaporate into the air. This will result in the sample having less mass than it did originally. This fact can be used to determine how many water molecules are attached to each formula unit in the hydrate.

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Example Suppose we have a 5.00 g sample of BaCl 2 * X H 2 O And we want to determine X. We would heat up our previously weighed sample until we are sure that all of the water has been evaporated. Then we reweigh our sample and see that it now only has a mass of 4.26 g. This tells us that we lost a total of 0.74 g of water.

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We now know that our original sample was 0.74 g water and 4.26 g of BaCl 2. We need to now convert these two numbers into moles. 4.26 g BaCl 2 x (1mol/208.23 g) = 0.0205 mol BaCl 2 0.74 g H 2 O x (1mol/18.02 g) = 0.041 mol We then set up our formula: 0.0205 BaCl 2 * 0.041 H 2 O Dividing by the smaller of the two numbers gives us: BaCl 2 * 2 H 2 O

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Materials Goggles Ring stand Crucible and lid Iron ring Clay triangle Bunsen burner Scale Tweezers MgSO 4 * X H 2 O

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Procedure Set up your ring stand, iron ring, and clay triangle. Weigh your crucible and lid, record your results in your data table Using weighing paper, weigh out about 3 g of MgSO 4 * X H 2 O and then put it in the crucible. Reweigh your crucible and lid with the sample inside, record your results in your data table Use subtraction to calculate the exact mass of MgSO 4 * X H 2 O in your crucible, record your results in your data table.

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Put your crucible now containing your sample carefully onto the iron ring with the lid slightly ajar. Call Mr. Berger over to light your Bunsen burner. Slowly wave the flame under the crucible to heat it slowly. We want to heat slowly to ensure that only water is being driven off. If you hear crackling or see sparks move the flame away IMMEDIATELY. After heating slowly for about 10 minutes turn off the burner. Using the tweezers CAREFULLY remove the crucible from the clay triangle, set it on the table with the lid COMPLETELY on. We want the lid on so that no water from the air can be absorbed by our now dehydrated sample.

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When your crucible is cool enough to touch, weigh it again with the lid on. Use subtraction to determine the mass of your dehydrated sample.

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Analysis Questions 1. Use the data you collected to calculate the formula for hydrated MgSO 4. Show all work. What would be the name of this compound? 2. How do the appearances of the hydrated and dehydrated crystals compare? How are they different? 3. If the hydrate’s formula is MgSO 4 * 7 H 2 O, what is the percent error in your experiment? What are some possible sources of error? What procedural changes could you make to reduce the error?

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Percent Error Calculation Percent error is calculated by subtraction your experimentally determined number from the actual number then dividing by the actual number. Example: If I do this experiment using MgSO 4 *7 H 2 O, and I determine that the amount of water molecules attached to each MgSO 4 is 6.98 my percent error is calculated as: [(6.5 – 7)/7] x 100 = - 7.1% (yes its ok to have a negative percent. That just means that your experimental number was lower that the actual number)

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Exp 5 – Percent Water in a Hydrated Salt

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