1.5: Experiment_605_Hydrates_1_2_1

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Student Name

Laboratory Date:

Date Report Submitted:

___________________________

Student ID

Experiment Number and Title

Experiment 605: Hydrates

Experiment 605: Hydrates

Section 1: Purpose and Summary

Determine the percent water of hydration in a hydrate sample.
Determine the number of moles of water, x, per mole of anhydrous salt and write the chemical formula of the hydrate sample.

Hydrates are ionic compounds that contain water molecules as part of their crystal structure. The bound water is called the water of hydration. Some compounds lose this water of hydration spontaneously (efflorescent) while some may require heating. On the other hand, an anhydrous salt (without water) can absorb water from the atmosphere and spontaneously dissolve in its own water of hydration (deliquescent).

A hydrate contains a definite number of water molecules bound to each ionic compound or anhydrous salt. In this experiment, students will dehydrate an unknown hydrate sample by heating, and calculate the amount of water lost in the process. Students will determine the ratio between the moles of water lost and the moles of anhydrous salt and write the chemical formula of the hydrate sample.

Section 2: Safety Precautions and Waste Disposal

Safety Precautions:

Use of eye protection is recommended for all experimental procedures.

A hot crucible will break if placed directly on a cold surface. Set hot crucibles on to wire screens to cool.

Waste Disposal:

When you are finished with the experiment, discard solid waste in the container marked ‘solid waste’ in the fume hood.

Section 3: Procedure

Part 1: Deliquescence and Efflorescence

Prepare two clean and dry watch glasses. Transfer a few crystals of sodium sulfate decahydrate, Na₂SO₄×10H₂O, into one watch glass, and anhydrous calcium chloride, CaCl₂, into the other. Observe each sample occasionally as you perform the rest of this experiment. Record your observations below.
Na₂SO₄×10H₂O	CaCl₂

Part 2: Formula of an unknown hydrate

Possible unknowns: CaCl₂×xH₂O, CaSO₄×xH₂O, AlK(SO₄)₂×xH₂O, MgSO₄×xH₂O

Obtain an unknown hydrate from your instructor. Record identification code for your unknown.	Identification code for unknown:
Assemble a wire triangle supported on an iron ring attached to a ring stand. Put a clean, dry porcelain crucible and lid on the wire triangle and heat over a Bunsen burner flame for about three (3) minutes to ensure complete dryness. An example setup is shown: $\\lmcpitpfs1.lmc.local\homedirs\pwest644\Documents\A_ZeroCostTextbooks\Chem_6_Rewrite_2018\In Process Experiments\2.jpg$ Allow the crucible to cool on the wire triangle. When the crucible is cool and safe to touch, weigh on an analytical balance. Record exact mass of the crucible and lid.	Mass of crucible and lid: ________________ grams
Transfer 2 – 4 grams of your unknown sample into the crucible and weigh again. Record exact mass. NOTE: You need to have both the total mass of the crucible, lid and sample as well as just the sample. You can determine the mass of the sample by subtraction of the mass of the crucible (a).	Mass of crucible, lid and sample (before heating): ________________ grams
Put the crucible and sample back on the wire triangle. Position the crucible such that it is at a slight angle on the triangle. Place the crucible lid so that the lid is slightly ajar. Gently heat the crucible for 5 minutes over a Bunsen burner flame (slowly move the burner back and forth across the bottom of the crucible).
After gentle heating for about 5 minutes, increase the heat applied to the crucible, lid, and sample until the bottom of the crucible turns red. Continue providing high heat for additional 10 minutes. Let the crucible cool for 5 to 10 minutes. Using crucible tongs, place the lid and the crucible on a wire gauze on the bench to finish cooling to room temperature.
When the crucible is cool and safe to touch, weigh on an analytical balance. Record exact mass. Be sure to include the lid with the crucible on the balance.	Mass of crucible, lid and sample (after final heating): ________________ grams
To ensure complete dehydration of sample, reheat the crucible, lid, and sample as in step #5, except to heat it for 5 minutes. Cool, and weigh again. Record exact mass. (Repeat this process, if necessary, until the mass of the sample is within 0.01 g of the previously recorded mass.)	Mass of crucible, lid and sample (after 2nd, etc. final heating): ________________ grams

Part 3: Reversibility of hydration

Transfer a small amount of solid copper (II) sulfate pentahydrate, CuSO₄×5H₂O, that just fills the bottom of a clean, dry 150-mm (medium size) test tube.

Using a test tube holder, grasp the test tube containing the hydrate and heat over a Bunsen burner flame while holding the test tube at a 45° angle.

While heating, closely observe the solid and the inside wall of the test tube. Record your observations.

When the solid residue seems to be completely dehydrated, allow the test tube to cool completely. Then, add a few drops of laboratory water to the solid in the test tube. What changes did you see? Record your observations.

Appearance of CuSO₄.5H₂O:

Appearance of solid after heating:

Appearance of the inside wall of the test tube after heating:

Appearance of solid residue after adding a few drops of laboratory water:

Section 4: Calculations

From the masses you recorded in Part 2 of this experiment, calculate the mass of the unknown hydrate sample.

(b) – (a)

Mass of the unknown hydrate sample:

(e) _______________ grams

Calculate the mass of the heated (dehydrated) sample. This is also called the anhydrous salt.

(d) – (a)

Mass of the heated sample (dehydrated):

(f) _______________ grams

From the mass of the hydrate and the mass of the heated sample, calculate the mass of water lost.

(e) – (f)

Mass of water lost:

(g) _______________ grams

Calculate percent water in the hydrate.

% water= $\frac{\text {mass of water lost (g)}}{\text { mass of hydrate (e)}}=$ x 100

Percent water in the hydrate:

Ask for the identity of the unknown hydrate sample from your instructor. Refer to a periodic table to obtain the molar mass of the anhydrous salt. Then, using this information and the mass of the heated sample, calculate the number of moles of the anhydrous salt.

Identify of the anhydrous salt: ________________________

Molar mass of anhydrous salt: ___________________g/mol

Show your equation here:

Moles of anhydrous salt:

(h) _______________ mol

Similarly, determine the molar mass of water. Then, calculate the number of moles of water lost from the sample.

Molar mass of water, H₂O: ___________________g/mol

Show your equation here:

Moles of water lost:

(i) _______________ mol

Calculate the ratio of moles of water lost to moles of anhydrous salt.

(i) ÷ (h)

$\frac{\text { mole of } H_{2} O}{\text { mole of anhydrous salt }}$ ratio

(j) ____ : ____

The ratio calculated (j) is expressed in the formula of the compound (hydrate). Write the chemical formula of the hydrated form of your unknown sample.

For example, if a given amount of hydrated copper(II) sulfate gave off 0.060 mole of H₂O and left behind 0.012 mole of anhydrous copper(II) sulfate, CuSO₄, then the ratio of H₂O to CuSO₄ is 5:1, and the formula would be written as CuSO₄×5H₂O.

Chemical formula of the unknown sample:

Post-lab Questions

Calculate the mass percent of water for the hydrate, LiNO₃×3H₂O.

What will be the probable effect if you kept the crucible completely covered during the entire heating and cooling processes? Would your calculated percent water in the hydrate be high, low, or unaffected? Explain.

If 2.752 g sample of Ca(NO₃)₂×xH₂O is heated to constant mass, the residue weighs 1.941 g. Determine the value of x and the formula of the hydrate.

Notes:

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