LAB 13: FORMULA FOR A HYDRATE EXPERIMENT
HONORS CHEMISTRY
Problem:
What is the percentage of water in a hydrated salt? What is the formula for the hydrate?
PRELAB:
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 an ANHYDROUS SALT. Here is an example of a hydrate:
CoCl2· 6H2O Cobalt(II) chloride hexahydrate
hydrate
heat
anhydrate
CoCl2
6H2O(s) --- CoCl2(s) + 6H2O(g)
In hydrates, 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. Notice, we use a middle dot to
connect the water units to the salt formula.
In this experiment you will measure the mass of a hydrated salt, 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 hydrated salt
and also the EMPIRICAL FORMULA (the simplest whole-number ratio of atoms in a compound) for the hydrated salt.
Pre-lab: Complete the table and answer the 3 prelab questions that follow – show your calculations
NAME OF SALT
CoCl2· ?H2O
MASS OF CRUCIBLE
MASS OF CRUCIBLE & HYDRATED SALT
MASS OF HYDRATED SALT
12.090 GRAMS
16.250 GRAMS
GRAMS
MASS OF CRUCIBLE & CONTENTS AFTER HEATING
MASS OF ANHYDROUS SALT
12.424 GRAMS
GRAMS
MASS OF WATER LOST
GRAMS
% WATER IN HYDRATED SALT
% WATER
MOLES WATER IN HYDRATED SALT
MOLES
FORMULA FOR ANHYDROUS SALT
MOLES ANHYDROUS SALT
MOLES
FORMULA FOR HYDRATED SALT
Pre lab questions:
1. What information is necessary to determine the percentage of water in a sample of a hydrate?
2.
How will the water be removed from the hydrate in this experiment?
3. A hydrate has the formula of Na3PO4 . 12H2O. What is the percent of water in the hydrate?
Materials:
Burner
Crucible
Crucible tongs ceramic pad
Clay triangle
Iron Ring
Hydrate
Ring stand
PREPARATION:
1. Clean and dry your crucible by gently heating it. When it is completely cooled, record
the mass.
2. Place a small sample of the hydrate into your crucible. Record the mass.
3. Place the crucible and contents in the clay triangle suspended in the iron ring as shown
EVAPORATION OF WATER:
1. Heat the crucible gently at first and then increase the flame to full intensity. Carefully stir the hydrate
to break it up and expose new surface area to the heat.
2. Shut off the flame and allow the crucible and contents to cool on the ceramic pad.
3. Measure and record the mass when completely cooled.
4. Repeat steps 1-3 of this part, until two successive masses are equal. This indicates that all the water
has been removed.
5. Complete the table by completing the necessary calculations
CLEAN UP:
1. Carefully clean and dry the crucible and crucible lid. Return it!
2. Place the anhydrous salts in the container in the hood.
3. Clean up your lab station, returning all equipment to its proper location.
4. Wash your hands!
Post lab questions:
1. What was the percent of water in your sample. Use your lab data.
2. What was the purpose of heating the hydrate until two successive masses were the same?
3. Why should the crucible, crucible lid and contents be cooled before finding their mass?
4. If the hydrate is not heated gently, what will result? How does this affect the final mole ratio when
writing the formula for the hydrate?
5. If given a 25.00 gram sample of this hydrate, according to your data, how many grams of water should
be driven off in this experiment?
6. Calculate the theoretical percent composition of water in Na3PO4*12H2O
7. Calculate your percent error % error = (exp-theor /Theor) x 100
Name of the hydrate
Mass of crucible (g)
Mass of crucible + hydrate (g)
Mass of the hydrate (g)
Mass of crucible and salt after
heating #1
Mass of crucible and salt after
heating #2
Mass of crucible and salt after
heating #3
Mass of crucible and salt after
heating #4
Mass of anhydrous salt (g)
Moles of anhydrous salt
Mass of water (g)
Moles of water
Mole ratio (Lowest whole number ratio)
Empirical Formula of the Hydrate
Formula for a hydrate rubric 67 pts
____ 2 pts
Lab is written according to basic lab instructions
____ 2 pts
Rubric is included
____ 5 pts
Questions answered with question clearly indicated with the answer
____ 10 pts Lab physically completed within one week of original date assigned
____ 8 pts
Prelab table completely filled in with answers circled and work shown. The factor label method
is utilized and proper sig figs and units are reported.
____ 2 pts
Prelab question 1 - clearly explains how the calculation is performed
____ 2pts
Prelab question 2 – clearly explains the reasoning for the specified procedure
____ 2 pts
Prelab question 3 – Clearly shows calculations to find the molar mass of the hydrate and the
water. Clearly shows the calculation for determining the percent of water in the hydrate. All
calculations include units of measure.
____ 14 pts Data table completely filled in with answers circled and work shown. The factor label method is
utilized and proper sig figs and units are reported.
____ 5 pts
Post lab question 1 - Calculation for the percent composition of water in the sample is shown
with units of measure and is calculated using lab data.
____ 2 pts
Post lab question 2 - Clearly states the purpose of heating the hydrate until two successive
masses were the same.
____ 2 pts
Post lab question 3 - Clearly explains the reason for allowing the crucible to cool before
massing. Clearly explains the affect of convection currents on the mass if the crucible is hot.
____ 2 pts
Post lab question 4 - Clearly explains the result of not heating gently. Explains the impact on
the formula of the hydrate.
____ 2 pts
Post lab question 5 - Calculation clearly shows the amount of water in a 25 gram sample of this
hydrate. Calculation should be based upon experimental percent of water found.
____ 2 pts
Post lab question 6 –Calculation for the theoretical percent composition of water in hydrated
salt.
____ 5 pts
Post lab question 7 - Calculation for the percent error is shown. Values are substituted into the
equation. Answer is rounded to the correct number of sig figs.