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Chemical Reactions: Water of Hydration - Laboratory 1 | CHEM 1405, Lab Reports of Chemistry

Laredo Community CollegeChemistry

Material Type: Lab; Class: Introductory Chemistry I; Subject: Chemistry; University: Laredo Community College; Term: Unknown 1989;

Typology: Lab Reports

Pre 2010

Uploaded on 08/18/2009

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LAREDO COMMUNITY COLLEGE
Science Department CHEM 1405 Laboratory 1
CHEMICAL REACTIONS: Water of Hydration
A. Short Overview
Many common substances exist as solid ionic salts that contain water molecules embedded in thier
crystal structures. This water is referred to as water of hydration, and can be removed from the crystals
by heating, or dehydration. Such water commonly gives the hydrated salts different properties, such as
color, than the anhydrous form. This is especially obvious when the metal cation is a member of the
transition metals.
In the first of four quantitative experiments involving chemical reactions (water of hydration, empirical
formulas, limiting reagents, and heat of reaction) that you will perform this semester, you will determine
the percent water present in an ionic salt. While this is an experiment some of you may have already done
in a high school chemistry class, it is a valuable experiment because it reinforces the concept of percent
composition.
In this experiment you will be provided with a sample of CuSO4•5H2O (used in plant food),
MgSO4•7H2O (Epsom salts), or FeSO4•7H2O, and asked to determine the experimental % water and the
actual composition of the solid based upon the amount of water removed from the sample. Each sample
can be identified by its color.
The chemical equations for the reactions are
CuSO4•5H2O (s)  CuSO4 (s) + 5 H2O (g)
FeSO4•7H2O (s)  FeSO4 (s) + 7 H2O (g)
MgSO4•7H2O (s)  MgSO4 (s) + 7 H2O (g)
B. Procedure
Exercise 1. Percent Composition of a Hydrated Salt
Chemicals: Hydrated Salts (CuSO4•5H2O (blue), FeSO4•7H2O (green), MgSO4•7H2O (white)),
Propane (C3H8)
Apparatus: evaporating dish, electronic balances, crucible tongs, scoop spatula, tripods, triangles,
wire gauze, bunsen burner, sandpaper
Safety Equipment: goggles, gloves, hood.
Objectives: In this experiment you will learn how to:
1. heat a sample in a crucible with a Bunsen burner.
2. use the method of “weight-by-difference” to determine mass quantities.
3. convert grams to moles.
4. determine the percent of water lost by heating a hydrated sample.
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LAREDO COMMUNITY COLLEGE

Science Department CHEM 1405 Laboratory 1

CHEMICAL REACTIONS: Water of Hydration

A. Short Overview

Many common substances exist as solid ionic salts that contain water molecules embedded in thier crystal structures. This water is referred to as water of hydration , and can be removed from the crystals by heating, or dehydration. Such water commonly gives the hydrated salts different properties, such as color, than the anhydrous form. This is especially obvious when the metal cation is a member of the transition metals. In the first of four quantitative experiments involving chemical reactions (water of hydration, empirical formulas, limiting reagents, and heat of reaction) that you will perform this semester, you will determine the percent water present in an ionic salt. While this is an experiment some of you may have already done in a high school chemistry class, it is a valuable experiment because it reinforces the concept of percent composition. In this experiment you will be provided with a sample of CuSO 4 •5H 2 O (used in plant food), MgSO 4 •7H 2 O (Epsom salts), or FeSO 4 •7H 2 O, and asked to determine the experimental % water and the actual composition of the solid based upon the amount of water removed from the sample. Each sample can be identified by its color. The chemical equations for the reactions are CuSO 4 •5H 2 O (s)  CuSO 4 (s) + 5 H 2 O (g) FeSO 4 •7H 2 O (s)  FeSO 4 (s) + 7 H 2 O (g) MgSO 4 •7H 2 O (s)  MgSO 4 (s) + 7 H 2 O (g)

B. Procedure

Exercise 1. Percent Composition of a Hydrated Salt

Chemicals: Hydrated Salts (CuSO 4 •5H 2 O ( blue ), FeSO 4 •7H 2 O ( green ), MgSO 4 •7H 2 O ( white )), Propane (C 3 H 8 ) Apparatus: evaporating dish, electronic balances, crucible tongs, scoop spatula, tripods, triangles, wire gauze, bunsen burner, sandpaper Safety Equipment: goggles, gloves, hood. Objectives: In this experiment you will learn how to:

  1. heat a sample in a crucible with a Bunsen burner.
  2. use the method of “weight-by-difference” to determine mass quantities.
  3. convert grams to moles.
  4. determine the percent of water lost by heating a hydrated sample.

Your instructor will assign you a hydrated salt. Each student will heat one sample. On the data sheet, the data for your sample is placed in column listed “Trial 1”. Your partner’s data is listed in column(s) “Trial 2” (and “Trial 3” if a group of three). Use the same electronic balance for each weighing. Tare the balance each time to 0.000 g before weighing the evaporating dish. CAUTION: Tie your hair behind your head.

  1. Obtain a Bunsen burner and tripod from the front of the lab. Place a wire triangle from your desk on top of the tripod.
  2. Clean your evaporating dish with soap and water, rinse with distilled water, and dry the evaporating dish with paper towel.
  3. Use your crucible tongs to place the evaporating dish on the triangle.
  4. Connect the Bunsen burner to the propane gas line with rubber hose. (CAUTION: Check the burner hoses carefully before lighting. Replace the burner hoses if they are old or cracked.) Turn on the gas and light the burner. Adjust the flame so that it is pale blue and does not appear yellow. (Yellow flames are caused by incomplete combustion and will leave soot on the evaporating dish; yellow flames are also cooler than blue flames.) Place the burner under the dish.
  5. Heat the evaporating dish until it is red-hot on the bottom for about 5 minutes. Use the tongs to place the dish on the desk, and allow it to cool to room temperature. (Caution: The evaporating dish will get very hot when heated. Do not touch the hot dish with bare hands.) Allow the dish to cool to room temperatures before weighing. The dish is cool enough to weigh when it feels cool as you pass your hand over it. Do not try to cool down the hot dish by running cold water over them; the porcelain dish will break. Also, do not set the hot dishes on your lab notebooks; they will burn through the notebook paper.
  6. Weigh the evaporating dish on the electronic balance to the nearest 0.001 g. Then use a spatula to add 2.5 – 3.0 grams of the assigned hydrated salt to the dish. Reweigh the evaporating dish to ± 0.001 gram.
  7. Replace the wire triangle with the wire gauze and place the evaporating dish on the wire gauze. Heat the evaporating dish gently for five minutes by placing the burner under the wire gauze.
  8. Replace the wire gauze now with the wire triangle and heat the dish strongly at red heat for an additional five minutes. Cool the crucible to room temperature and weigh the evaporating dish. The dish should weigh less than it did in step 7 due to the loss of water.
  9. Reheat the evaporating dish on the wire triangle with a strong flame for another five minutes, cool and reweigh. The weight after the two heating/cooling increments should agree within 0.005 g. If not, repeat the heating process for three additional minutes, followed by cooling and weighing.

Trial 1 Trial 2 Trial 3 Unknown number (^) 18 48 27 Mass of empty crucible & cover (^) 18.245 21.882 18.625 g Mass of crucible, cover, and hydrated salt 19.965 23.954 20.593 g Mass of hydrated salt (^) 1.720 2.072 1.968 g Mass of crucible, cover, & dehydrated salt, final heating (^) 19.324 22.907 19.770 g Mass of dehydrated salt 1.079 1.025 1.145 g Mass of water lost (^) 0.641 1.047 0.823 g Moles of water lost (^) 0.035581 0.058118 0.045684 mol Formula of dehydrated salt (^) CuSO4 MgSO4 FeSO Molar mass of dehydrated salt (^) 159.6 120.37 151.94 g/mol Moles of dehydrated salt (^) 0.00676 0.00852 0.00754 mol Moles water ÷ Moles dehydrated salt (^) 5.26 6.83 6. % water (experimental) (^) 37.3% 50.5% 41.8% % % water in hydrated salt (theoretical) (^) 36.1% 51.2% 45.4% % error (^) 3.3% 1.2% 7.8% %