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
• CuSO4.5H2O
• BaCl2.2H2O
• Na2HPO4.12 H2O
4 mol H2O/mol CaCl2
5 mol H2O/mol CuSO4 (blue)
2 mol H2O/mol BaCl2
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.
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
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
– CoCl2.2H2O
– CoCl2
CoCl2.6H2O
red
6 mol H2O/mol CoCl2 (red) (hexahydrate)
2 mol H2O/mol CoCl2 (violet) (dihydrate)
no H2O, anhydrous (blue)
D
-4 H2O
CoCl2.2H2O
violet
D
-2 H2O
CoCl2
blue
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.
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-thetouch 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 0.001 g of each other
• Report the weights on the lab report sheet
and determine the average of your
weighings
Exp 5 – Percent Water in a Hydrated Salt
Part A (continued):
1.
Average mass of crucible+lid
2.
Add about 2.0 –2.5 g sample into the fired cool crucible
3.
Weigh crucible + lid + hydrate sample (0.0001 g precision)
4.
Mass of hydrate sample = (crucible + lid + hydrate sample ) – (crucible+lid)
Part B: Thermal decomposition product of sample
1.
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
2.
Repeat heating for 2 min, and cool crucible (“warm to touch”)
Weigh crucible + lid + residue (0.0001 g precision)
Record as Final mass if the difference from previous weighing
is not more than + or - 0.010 g
Mass(g)
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
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
_
CORRECTION:
• USE an amount of sample ~ 1.5 to 2.0g for
a narrow crucible
• USE 2.0-2.5 g for a wider-mouth crucible
Next week
Due Monday July1, 2013
Exp 5 done today
• Report sheets p. 83-84
• Questions #1-7 on p. 84
Next week’s Exp 2 p.53:
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Prelab assignment: answer
Read and understand:
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–
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Goal of Experiment
Experimental Procedures
Safety Precautions for this lab