Waters of Hydration

1
Waters of Hydration
2
Experimental Objective

• Determine the formula of a hydrate from
  experimental data

3
Hydrate

• Anhydrous salt
• Waters of hydration

Copper (II) Sulfate Pentahydrate CuSO45H2O
4
Experimental Overview

• Obtain mass of hydrate
• Remove waters of hydration
• Oven dry
• Crucible dry
• Obtain mass of anhydrous salt

5
Remove waters of hydration Heat to a constant
mass

• Heat sample (oven or crucible)
• Cool sample
• Obtain mass of sample
• Repeat steps 1-3 until the mass loss between
  cycles is less than 0.003 g.

6
Remove waters of hydration oven Start this
FIRST

• Mark 50 mL beaker with initials
• Obtain mass of dry, CLEAN 50 mL beaker
• DO NOT touch with fingers
• Mass of paper clip thick layer of assigned
  hydrate in 50 mL beaker (1 gram)

7
Remove waters of hydration oven

• Place beaker with hydrate in glass Pyrex dish
  next to appropriate oven
• 115o
• ammonium oxalate
• barium chloride
• 200o
• cobalt (II) chloride
• calcium sulfate
• nickel (II) chloride
• manganese chloride
• Heat for 30 minutes
• Cover cooling beaker with watch glass

8
Remove waters of hydration Crucible Start this
while oven sample is heating

• Prepare crucible
• Heat sample

9
Prepare Crucible

• Heat strongly for 5 minutes
• Handle hot crucible with tongs.
• DO NOT TOUCH with fingers.
• Burns
• Skin oils
• Obtain combined mass of cool crucible and lid.
• Obtain mass of lid only

10
Remove waters of hydration - crucible

• Place 1 gram of hydrate in crucible
• Do not use significantly more or less than 1 g.
• Determine mass of hydrate, crucible and lid.
• Heat gently (brush) with lid partially on until
  no visible evidence of water remains
• Color (sometimes)
• boiling
• Heat with medium flame 5 minutes
• Heat with high flame 10 min
• DO NOT overheat
• Cover while cooling
• Repeat 4-6 until constant mass obtained

11
Dispensing solid reagents

• ALWAYS replace the lid of a jar or bottle after
  dispensing a reagent. 
• Clean scoopula between dispensing substances.
• Never return excess reagent to reagent jar.

12
Use of the Electronic Balance

• Doors closed
• Sample cool
• Remove the weigh dish from the balance to add or
  remove substance

13
Data AnalysisDetermine the formula of the hydrate

• Determine the mass of the waters of hydration
• mass of waters of hydration mass of hydrate
  mass of anhydrous salt
• Convert the mass of anhydrous salt to moles of
  anhydrous salt.  The identity of the anhydrous
  salt must be provided by the lab instructor. 
• Moles of anhydrous salt (mass of anhydrous
  salt)/(molar mass of anhydrous salt)
• Convert the mass of the waters of hydration to
  moles of waters of hydration.
• Moles of waters of hydration (mass of waters of
  hydration)/ (18.01 g/mole)
• Determine the ratio of moles of water of
  hydration to moles of anhydrous salt.
•  
• Moles of waters of hydration/moles of anhydrous
  salt
• Round ratio to an integer
• Round ratios ending in 0.3 or greater UP

14
Next Week Stoichiometry of a Reaction

• READ procedure in lab manual
• READ/STUDY WebCT related material
• COMPLETE WebCT pre-lab assignment 24 hours prior
  to lab