Percent Hydrate Lab 2020 PDF

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Percent Composition of a Hydrate Lab

Introduction: Ionic compounds often separate from water solution with molecules of water incorporated into the solid. Such compounds are referred to as hydrates. Hydrates are compounds that have a specific amount of water trapped inside its crystals, called water of hydration. In writing the formula of a hydrate, a dot connects the formula of the compound with the number of water molecules per formula unit. The number of water molecules is always a whole number. Several examples include sodium carbonate decahydrate (Na2CO3 • 10H2O), calcium sulfate dihydrate (CaSO4 • 2H2O) and magnesium sulfate, also known as Epsom salt (MgSO4 • 7H2O). In writing the formula, a dot is used to separate the formula of the ionic compound from that of water and is viewed as an addition sign in determining molar mass. Hydrates will lose all or part of their water of hydration when exposed to air or heated. Frequently, this dehydration is accompanied by a colour change. The remaining ionic compound is known as the anhydrous version of the compound. Crystals are dry to the touch. They are unchanged in composition or appearance in normally moist air. When crystals are heated above 100 oC, they lose their water of hydration. The forces holding the water molecules in hydrates are not very strong, so the water is easily lost and regained.

Objectives: 

To find the percent of water of a hydrate.



To determine the empirical formula of a hydrate.

To determine what percent by mass of a hydrate is water, first determine the mass of water in one mole of hydrate. Then determine the molar mass of the whole hydrate (ionic compound plus water). percent by mass H2O =

mass of water x 100% mass of hydrate

Pre-lab:

(Show all work and necessary units) 1. In a minimum of one (1) paragraph summarize the introduction, copy materials, procedures and complete the following questions. 2. Calculate the molar masses of the three hydrates listed in the introduction. Show all your work.

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Percent Composition of a Hydrate Lab 3. Calculate the percent of the water in sodium carbonate decahydrate. 4. How much anhydrous Na2CO3 would remain behind after strongly heating 25.0 g of Na2CO3 • 10H2O? (Hint: Use the answer from the previous question and assume all water is lost.) 5. A 15.67 g sample of a hydrate of magnesium carbonate was heated to drive off the water. The mass of the resulting anhydrate was 7.58 g. What is its molar mass and calculate the formula of the hydrate (it must be MgCO3·nH2O)? 6. Copy all data charts for the lab.

Materials: Bunsen burner Rubber hose Spatula Balance crucible & cover iron ring ring stand copper (II) sulfate hydrate weighing boat

Procedure: 1. Setup the ring stand with iron ring and ring. Place the clay triangle over the ring to hold the crucible. 2. Measure and record the mass of a clean, dry, empty crucible. 3. Measure out 2 to 3 g of the magnesium sulfate in the crucible. Record the mass. Place the crucible in the clay triangle. 4. Light the burner with a flame that is approximately 3 to 4 cm high (1.5 to 2 inches for the imperialist) and position the flame under the crucible so that the inside blue cone is just below the crucible. While heating, be ready to adjust the height or remove the burner in case of excess spattering. 5. Heat the hydrate for 5 to 10 minutes and allow for cooling. Take the mass of the crucible and contents and record the result in trial 1 of the observation table. 6. Repeat steps 4 and 5 until a consistent mass is obtained. 7. Record any qualitative observations (i.e. spattering, spilling, smoke). 8. Clean up lab area ( point will be deducted if area is not properly cleaned )

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Percent Composition of a Hydrate Lab Data & Observation: Mass of Crucible (empty) Mass of Crucible and Hydrate (before heating) Mass of Crucible and Anhydrous after heating: 1st heating 2nd heating 3rd heating

Qualitative Observation:

Analysis Questions: (Feel free to print off analysis questions, answer and place inside your lab notebook or you may write and answer all questions in your lab notebook. Complete during and after you complete your lab.

1. Calculate the mass in grams of the original hydrate used in the experiment.

2. Calculate the mass of water lost during heating.

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Percent Composition of a Hydrate Lab

3. Calculate the percent composition of water and the anhydrous in the hydrate.

4. The accepted value for the percent water in magnesium sulfate hydrate is 51.2%. Determine your percent error. % error =

Therectcal Yeild −Actual Yeild ActualYeild

x 100%

5. Use your answer to #2 above to determine how many moles of water were removed from the hydrate.

6. Calculate the mass of anhydrous compound that remained after heating.

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Percent Composition of a Hydrate Lab

7. The anhydrous compound is MgSO4. Use your answer to #6 to find the number of moles of MgSO4 in your sample.

8. Find the ratio of moles of water to moles of magnesium sulfate in your compound by dividing your answer to #5 by your answer to #7.

9. Find the complete formula for the hydrate from your answer to #8. Hint: the formula is MgSO4 · nH20

10. Describe two possible sources of experimental error in the procedure.

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Percent Composition of a Hydrate Lab 11. Describe and explain two changes you will observe or measure if the anhydrous is left on the lab bench for a few days.

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