Formula of a Hydrated Salt
Introduction:
A hydrate is a compound that contains water in its crystal structure. The water may be removed
from the salt in the laboratory by heating the salt. The salt without the water is called a dry or
anhydrous salt.
In a hydrate the water molecules are a distinct part of the compound but are joined to the salt by
connections that are weaker than the connections in the salt or the connections in the water molecules.
In this experiment you will measure the mass of a hydrated salt, MgSO4 ▪ 7H20 magnesium
sulfate, then remove the water from the crystals and measure the mass of the anhydrous salt. The data
gathered will allow you to determine the percent water in the hydrate salt and also the empirical formula
for the hydrated salt. Notice we use a dot to connect the water units to the salt formula.
Purpose:
To define a hydrate. To describe what the formula of a hydrate shows.
To experimentally determine the formula of a hydrated salt.
Safety
1. Hot items look the same as cool items. Be sure to wait until the crucible is cool
before transferring it from place to place.
2. Wear laboratory safety goggles and a lab apron at all times in the laboratory.
Procedure:
Part 1
Preparation
1. Clean a porcelain crucible and dry it by gently heating it over a Bunsen burner,
see Figure 1, page 3. Record the mass of the cool and dry crucible in Table 1.
2. Add 3-5 g (one good scoop!) of Magnesium Sulfate ▪ heptahydrate
(MgSO4 ▪ 7H20) to the crucible and record the total mass in Table 1.
3. Place the crucible and hydrate in the clay triangle apparatus to be heated.
Part 2
1.
2.
3.
4.
Evaporation of Water
Heat the crucible gently over the Bunsen burner for ten to twelve minutes.
Shut off the flame and allow the crucible to cool completely.
Record the mass of the crucible and its contents in Table 1.
Repeat steps 1 – 3 until at least two successive masses are equal within the
uncertainty of the balance. This indicates that all of the water has been removed
from the crystals.
Cleaning up
1.
2.
3.
4.
Carefully clean the crucible and dry it. Return it to its proper location.
Place sample of hydrate in the container provided for solid waste.
Clean up the lab station putting all equipment to its proper location.
Wash your hands thoroughly before leaving the laboratory.
Data Presentation
TABLE 1. Hydrated Salt Data
Mass of crucible
Mass of crucible + hydrated salt
Mass of crucible and contents after heating #1
Mass of crucible and contents after heating #2
Mass of crucible and contents after heating #3
g
g
g
g
g
Data Analysis **(Show all calculations)**
TABLE 2. Hydrated Salt Data
1) Molar mass of hydrated salt (MgSO4 ▪ 7H20)
g/mol
2) % comp by mass of water in hydrated salt
% water
3) Mass of hydrated salt before heating
g
4) Moles of hydrated salt
mol
5) Molar mass of anhydrous salt (MgSO4)
g/mol
6) Mass of anhydrous salt after heating
g
7) Moles anhydrous salt
mol
8) Mass of water evaporated
g
9) Moles water evaporated
mol
10) mass of anhydrous salt after heating + mass
g
of water evaporated
11) Experimental coefficient for the moles H2O
12) Experimental formula for hydrated salt
13) Molar mass of experimental formula
g/mol
14) Molar mass % error
15) Mass % error
1. Use the periodic table to determine molar mass of the hydrated salt
(MgSO4 ▪ 7H20) and record in Table 2, line 1.
2. Determine the % composition by mass of water in the hydrated salt (MgSO4 ▪ 7H20).
Record in Table 2, line 2.
3. From Table 1 data, determine the mass of hydrated salt before heating and record in
Table 2, line 3.
4. Use the molar mass of hydrated salt from line 1 to convert the mass from line 3 to
moles of hydrated salt before heating and record in Table 2, line 4.
5. Use the periodic table to determine molar mass of the anhydrous (dry) salt (MgSO4)
and record in Table 2, line 5.
6. From Table 1 data, determine the mass of anhydrous (dry) salt after heating and
record in Table 2, line 6.
7. Use the molar mass of anhydrous salt from line 5 to convert the mass from line 6 to
moles of anhydrous salt and record in Table 2, line 7.
8. Subtract the mass of the anhydrous salt (line 6) from the mass of hydrated salt (line 3)
to determine the mass of water that was evaporated during heating and record in
Table 2, line 8.
9. Use the molar mass of water to convert the mass of water evaporated from line 8 to
moles of water evaporated and record in Table 2, line 9.
10. Add the mass of anhydrous salt after heating (line 6) to the mass of water evaporated
(line 8) and record in Table 2, line 10.
11. Divide the number of moles of water evaporated (line 9) by the number of moles of
anhydrous salt (line 7) to determine the experimental coefficient for the moles H2O.
Enter this number in Table 2, line 11.
12. Write the experimental formula for the hydrated salt by placing the number from line
11 as the coefficient for H2O in the following hydrated formula: MgSO4 ▪ __H20.
Enter this formula in Table 2, line 12.
13. Calculate the molar mass of the experimental formula (line 12) and record it in Table
2, line 13.
14. Calculate molar mass percent error and record in Table 2, line 14.
[Molar mass of hydrated salt (line 1)–Molar mass of exp formula (line13) ] X 100
[Molar mass of hydrated salt (line 1)]
15. Calculate the mass percent error and record in Table 2, line 15.
{Mass of hydrated salt before heating-(mass of anhydrous salt after heating + mass of water evaporated)} X 100
Mass of hydrated salt before heating
{(line 3)-(line 10)} X 100
(line 3)
16. Be sure to show the work for all steps that require calculations. A neatly written page
including units appended to the back of your report is acceptable.
Conclusions
1.
2.
3.
4.
5.
Why should the crucible and contents be cooled before finding its mass?
Why must at least two successive mass readings be equal before finishing the experiment?
What is probably the most common source of error for % error calculation in step 14.
What is probably the most common source of error for % error calculation in step 15.
Predict and explain what would most likely happen to the mass of the anhydrous salt if it
were to be left exposed to the atmosphere?
Figure 1
Apparatus for heating a hydrate