1. Exp 5 – Percent Water in a Hydrated Salt
Background
In chemistry, a neutral ionic compound is typically referred to as a “salt”
Hydrates are inorganic salts that contain a specific number of water molecules crystallized with the salt (“water of crystallization” or “water of hydration“)
Examples are
CaCl2.4H2O 4 mol H2O/mol CaCl2
CuSO4.5H2O 5 mol H2O/mol CuSO4 (blue)
BaCl2.2H2O 2 mol H2O/mol BaCl2
Na2HPO4.12 H2O 12 mol H2O/mol Na2HPO4
Most hydrates have simple formulas with a ratio of 1 salt: xH2O,but it is not always the case
cadmium sulfate hydrate is best represented as (CdSO4)3.8H2O
When determining the formula of a hydrate you must not assume that it is one with a simple formula.

2. Exp 5 – Percent Water in a Hydrated Salt Adsorption and Absorption
Salts attract water from air and gain (some) weight
Adsorption of H2O on crystal surface
Some salts incorporate H2O in crystal structure
Absorption of H2O
Definite amount of H2O as part of crystal structure
Hydrates or Hydrate Salts

3. Exp 5 – Percent Water in a Hydrated Salt
Hydrates can normally be decomposed into the anhydrous (without water) salt and water by gentle heating.
From mass data collected the number of molecules of hydrated water can be determined per molecule of anhydrous salt.
Heating removes H2O gradually
yields anhydrous form
CoCl2.6H2O 6 mol H2O/mol CoCl2 (red) (hexahydrate)
CoCl2.2H2O 2 mol H2O/mol CoCl2 (violet) (dihydrate)
CoCl2 no H2O, anhydrous (blue)
CoCl2.6H2O CoCl2.2H2O CoCl2 D D
violet
-4 H2O
-2 H2O
red
blue

4. Exp 5 – Percent Water in a Hydrated Salt
Some salts spontaneously lose water molecules to the atmosphere: efflorescent
Some salts spontaneously absorb water from the atmosphere: deliquescent
Determinations carried out by measurements of mass throughout the experiment are referred to as gravimetric analysis
Thus, the determination of % by mass of water in a hydrated salt uses gravimetric analysis.

5. Exp 5 – Percent Water in a Hydrated Salt
Part A: Sample Preparation
Heat a crucible + lid on a clay triangle in a the flame of a lab burner for about 5 minutes (as you did for Exp. 7)
Let the crucible+lid cool to warm-to-the-touch and weigh. Record in 4 decimal points
Reheat the crucible+lid to make sure the crucible is really dry and repeat the weighing as above
Repeat until two consecutive weightings are within g of each other
Report the weights on the lab report sheet and determine the average of your weighings

6. Exp 5 – Percent Water in a Hydrated Salt
Part A (continued):
Average mass of crucible+lid Mass(g)
Add about 2.0 –2.5 g sample into the fired cool crucible
Weigh crucible + lid + hydrate sample ( g precision)
Mass of hydrate sample = (crucible + lid + hydrate sample ) – (crucible+lid)
Part B: Thermal decomposition product of sample
Heat the crucible gently for 2-3 min with lid slightly ajar
Continue heating at full flame for 10 min
Cool crucible to “warm to touch” and weigh
Repeat heating for 2 min, and cool crucible (“warm to touch”)
Weigh crucible + lid + residue ( g precision)
Record as Final mass if the difference from previous weighing
is not more than + or g

7. Exp 5 – Percent Water in a Hydrated Salt
Calculations:
#4. Percent by Mass of volatile water
% by mass= mass water x 100%
mass hydrated salt
#6. Standard Deviation* of % H2O:
Measures the spread of your results (how far they are from each other)
#7. Relative Standard Deviation* (%RSD):
Measures the spread of the individual results compared to the average result

8. Exp 5 – Additional Information
Why is it important to apply heat only as instructed?
The salt itself could decompose into other compounds
Can correct % calculations still be obtained?
Yes but you must know the formulas of the possible
decomposition products and do extra calculations _

9. CORRECTION:
USE an amount of sample ~ 1.5 to 2.0g for a narrow crucible
USE g for a wider-mouth crucible

10. Next week Due Monday July1, 2013 Exp 5 done today
Report sheets p
Questions #1-7 on p. 84
Next week’s Exp 2 p.53:
Prelab assignment: answer
Read and understand:
Goal of Experiment
Experimental Procedures
Safety Precautions for this lab