Nick DiPreta
Jonny Masci
Composition of Hydrates
Due: 2/7/10
Pre-Lab summaryHydrates are ionic compounds that have a certain amount of water as a part of their structure.
This water is chemically combined with a salt in a definite ratio. Ratios depend on the different
hydrates and are specific for any given hydrate.
The formula of a hydrate is represented in a very specific way. The hydrate in this experiment
was MgSO4∙xHOH. The salt unit always comes first, and the water formula comes next. The
dot means that the water is boned loosely to the salt. The “x” stands for the number of
molecules of water bonded to one salt unit. This formula illustrates the law of “definite
composition”.
When hydrates are heated, the “water of hydration” is released as water vapor. The remaining
solid is called the anhydrous salt. The reaction for heating a hydrate is:
Hydrateanhydrous salt+water
The percentage of water in a hydrate can be found experimentally by accurately
determining the mass of the hydrate and the mass of the anhydrous salt, by heating the
hydrate.
The formula for percentage water is %HOH= (100) (Mass HOH/Mass Hydrate)
Purpose: Thus, in this experiment, we will determine the percentage of water in a hydrate, as
well as learn and understand the formulas and composition of hydrates and the law of definite
composition.
Equipment:
Evaporating dish, crucible tongs, microspatula, lab balance, ring stand, wire gauze, lab burner,
safety goggles
Materials:
Magnesium sulfate hydrate MgSO4∙xHOH
Safety
Do not touch a hot evaporating dish with your hands. Tie back long hair and secure loose
clothing when working with the open flame. Note the caution alert symbols in the procedure
and be sure to wear safety goggles at all times.
Procedure
1. Prepare the Bunsen burner and ring stand set up shown on page 58 in figure 11-1.
2. Heat the dish with the hottest part of the flame for three minutes.
3. Using crucible tongs, remove the evaporating dish from the apparatus. Place it on an
insulated pad and allow it to cool for several minutes.
4. Find the mass of the evaporating dish to ±0.01 grams. Record the mass in the observations
and data section.
5. With the evaporating dish on the balance, measure into it 5-10 grams of the magnesium
sulfate hydrate. Record the data.
6. Place the evaporating dish and the hydrate on the wire gauze. Gently heat he dish by moving
the burner back and forth around the base. Increase the heat gradually. Avoid any
popping and spattering.
7. Heat strongly for 25 minutes. During heating, a microspatula may be used to “spread” the
solid and break up any “caked” portions of the hydrate. Be careful not to pick up any of
the solid on the microspatula. If the edges of the solid appear to be turning brown,
remove the heat momentarily and resume heating at a gentler rate.
8. Allow the evaporating dish to cool for about a minute. Immediately find the mass of the dish
and anhydrous salt, and record the data.
Observations and Data
Object
Mass of evaporating dish
Mass of evaporating dish+ hydrate
Mass of evaporating dish + anhydrous salt
Calculations
1. Find the mass of the hydrate used (b-a).
46.83g-39.23g= 7.6g
2. Find the mass of the water lost (b-c).
46.83g- 43.4g= 3.43g
Mass (grams)
39.23
46.83
43.4
3. Find the percentage of water in the hydrate: The formula for percentage water is %HOH=
(100) (Mass HOH/Mass Hydrate)
45.13%= (3.43g/7.6g)= 0.4513 X 100= 45.13%
Conclusions and Questions
1. The true value for the percentage of water in this hydrate is 51%, what is your experimental
error?
Experimental error=
Where expt.#= experimental value and std#= standard value
(51-45.13)/51=0.12 X 100= 12%
2. Why must you allow the evaporating dish to cool before measuring its mass?
One must allow the evaporating dish to cool before measuring its mass so that the heat from
the evaporating dish does not break the electrical balances sensors.
3. Why must you measure the mass of the anhydrous salt immediately upon cooling?
One must measure the mass of the anhydrous salt immediately upon cooling in order to avoid
measuring a value of the anhydrous salt after it has taken in water. If one waits too
long, it will begin to absorb the water that it has lost in the heat reaction.
4. Explain how your class’s results in this experiment support the law of definite composition.
"The Law of Definite Composition states: A compound always contains two or more elements
combined in a definite proportion by mass ". All of the students in our class obtained
different values for the mass of the hydrate and the anhydrous salt, but calculated the
same formula for the hydrate in the end. By calculating the same formula for the
hydrate, the law of definite composition is proved, as it shows that the hydrate is in a
definite composition.
5. Given the true mole masses of MgSO4 (120g) and HOH (18g), how could you find the exact
formula of the hydrae from your experimental data? That is, how could you find the
formula MgSO4∙xHOH?
7.60g (mass of hydrate) – 3.43g (mass of water in hydrate)= 4.17g (mass of anhydrous salt)
4.17g (mass MgSO4)/ 120 (true molar mass)= 0.03475 mol MgSO4
3.43 (mass HOH)/ 18 (true molar mass)= 0.191 mol HOH
0.03475 (mol MgSO4) / 0.03475mol (MgSO4)= 1 mol in formula
0.191 (mol HOH)/ 0.03475 mol (MgSO4)= 5.5 mol in formula
X= 5.5
Conclusion
By heating a hydrate, we can eliminate most, if not all of the water from it, and determine the
percentage of water in a hydrate. Also, it is possible to calculate the formula of the
hydrate by obtaining a ratio of the water to the anhydrous salt in the hydrate.