PERCENT WATER IN A HYDRATE
OBJECTIVE: Determine °/owater bound in selected
BACKGROUND:
A hydrate is a salt that has crystalized from aqueous solution with weakly bound water molecules contained in the
crystal lattice structure. Water in selected hydrates is bound into the molecular structure in definite stoichiometric
amounts. The water of the hydrate can be removed by heating the sample producing the owhydrous (no water) salt.
Examples of salt hydrates include Crc13.6H20, CoC12.6H20, Nis04.7H20, CuS04.5H20 and Bec12.2H20. The formula
notation used indicates that a specified number of water molecules is attached to the geometric structure of the salt. For
stoichiometric calculations using the hydrated salt, the formula weight is determined by adding the atomic weights
making up the salt plus the indicated number of water molecules times the formula weight of water. For example, the
formula weight of CuS04.5H20 is determined
as follows:
Formula wt CuS04.5HzO = Cu + S + 40 + 5(H20)
= 1(63.54 amu) + 1(32.064 amu) + 4(15.9994 amu) + 5(18.01 amu)
= 249.965 amu a 250 amu
ln this experiment, the weight percent of water in a hydrate will be determined. The water of the hydrate is weakly
bound to the salt's molecular structure and may be easily removed by heating a measured sample in a crucible over a
Bunsen Burner.
From the weight of the hydrate salt before heating and the anhydrate salt after heating, the amount of
water released can be determined and subsequently the percent water by weight. The following formula is used:
%Water =
Wt. of HOH Removed
00%
(Eqn.1)
Wt. of Salt Hydrate
The accuracy and precision of this analysis depends upon heating the sample sufficiently to drive off all of the bound
water. However, care must be taken not to over-heat the sample and degrade the sample. Some hydrate salts will
require stronger heat applications while others will require less heat application. One method of controlling the heating
rate is to heat the sample of interest in short timed intervals until achieving a constant target weight of sample. This
'theoretical' target weight can be determined from theoretical mass considerations. Knowing the mass of hydrate used
and the theoretical percent composition, the amount of anhydrate can be determined using the following formula:
Mass ofAnhydrate = Mass of Hydrate -[(theoretical decimal fraction of water)(Mass of Hydrate)I
(Eqn. 2)
The `target mass' of crucible containing sample is then determined by adding the empty crucible weight to the
theoretical mass of anhydrate. Heating in small timed intervals followed by weighing on analytical balance will allow for
consistently accurate and reproducible results.
EXPERIMENTAL
After determining the empty weight of a clean porcelain crucible, measure a small sample of hydrate salt into the
crucible. After determining the mass of crucible + hydrate, determine the mass of hydrate in the crucible by subtracting
the crucible empty weight. Based upon the theoretical percent water value and the mass of hydrate, calculate the
theoretical weight c)f the anhydrate using equation 1 shown in the background dialogue. Add to the anhydrate weight
the empty weight of the crucible to get the theoretical target weight. Heating in predetermined timed intervals allows
control of heating as the water of hydrate ls removed from the hydrate salt. Heating proceeds until a crucible + contents
value is close to the target theoretical value. This is the `last best heating' mass of crucible and now anhydrate sample in
the crucible. From this value, determine the mass of water experimentally removed by heating. That is, mass of water
removed = hydrate weight -anhydrate weight. The %Water in the test sample is then calculated using equation 1
shown in the background section.
Determination of %Water in a Salt Hydrate
DATA (Group
Wt. (g)
=>
A = Crucible Empty
=>
8 = Crucible + Hydrate
=>
C = Hydrate
D = Anhydrate (theoretical)=(Hydrate)-(WaterinHydrate)=(Hydrate)-O.36(Hydrate)
=>
=>
E = Crucible + Anhydrate (Theoretical)
F = Crucible + Contents (Heating Series)
1
Heat in short time intervals until closetocalculatedtheoreticalvalue(Suggeststartingwith10sec.Intervals)
2
3
4
5
6
G = Anhydrate (Actual) = [(Best F)-A]
H = Wt. of H20 removed (Experimental)=Hydrate-Anhydrate=C-G
=>
-_>
H = %H20 = (H/C)100%
DATA TREND ANALYSIS
Test #
1
2
3
4
5
Average
STDEV
Cl = 2(STDEV)
R(low)
R(High)
%HOH
Exp. 1
Exp. 2
Exp. 3
Exp. 4
Exp. 5
Conclusion:
The experimental %HOH in CuS04.5H20 was found to be
range of reliability (99%) from
represents a % Error of ±
%to
% giving a
% based upon 5 test trials. This
% based upon an accepted value of 36% w/w obtained
from theoretical determination of %Water in a hydrate using formula mass values.
Discussion:
The removal of the substrate water from the salt hydrate CuS04.5H20 requires careful heating
in timed intervals until the crucible and sample weight approaches the theoretical value of 36%
w/w using formula weight values. For CuS04.5H20 removal of the H20 substrate is relatively
easy but must be managed in the timed intervals to minimize degradation of sample. For other
hydrates, the same technique can be used but because of variations in physical bond strengths
the timed intervals needed may be different than noted in this study.
For this study, excellent trend results are obtained if the rate of heating is carefully managed.
Copper (11) Sulfate Pentahydrate is heat labile and one should be aware that rapid or extended
application of heating will degrade the test sample and result in poor experimental trends.