Lab 24
The Hydrate Lab
Hydrates are ionic compounds that have water molecules trapped in their crystal structure. Since all
ionic compounds are also salts, we could just as easily have said that hydrates are salts with water molecules
trapped in their crystal structure. The trapped water can be driven out of the crystal with careful heating,
leaving the dried salt behind. This dried salt is more formally known as the anhydrous salt. Each hydrate has a
uniform number of water molecules attached to every unit of salt. This amount of water molecules per unit of
salt is always (for you) a whole number.
Step 1. Read the general lab procedure below. It will be used for each of the hydrate labs.
A major concern is the breakage of crucibles and lids. Take whatever precautions are necessary to
insure that breakage is at a minimum. In addition, keep the crucibles dry. When not in use, store them in a
desiccator. After use, gently scrape out the contents of the crucible into the waste containers. If you need to
dry a crucible or lid, 2-5 minutes over a burner flame will suffice.
Find the mass of your empty crucible and lid. Add a few grams of hydrate (between 2.00 and 5.00
grams) to your crucible, then weigh the crucible, lid and hydrate. Set the crucible on a clay triangle, on a ring
attached to a ring stand.
Heat the crucible with the lid slightly ajar to drive of the water from the hydrate. Heat gently at first to
break water out of the crystal, then only a little more strongly for a few minutes until the water appears to be
gone. Then heat strongly for about two minutes to drive traces of water from the center of the mass of hydrate.
Let the crucible cool and then weigh the crucible, lid and anhydrous salt. Reheat the crucible for two
minutes on the burner, cool, and weigh again.
Scrape out the contents of the crucible and discard into waste beakers. Return the crucible and lid to the
desiccator. Calculate the masses of hydrate, anhydrous salt and water.
Step 2. Part A: Perform the hydrate lab using copper (II) sulfate pentahydrate (CuSO4 5 H2O) using the
general procedure outlined above. Do one trial for each group member and perform the calculations.
Step 3. Enter your mass of hydrate, anhydrous salt and water from the copper (II) sulfate hydrate lab into the
hydrate lab A spreadsheet (worth 10 pts. if you enter it by the end of the second day of lab and zero points if
you are late).
Step 4. Solve these two problems:
1) How are the experimental masses you obtained in lab related to the molar masses on the periodic
table? Note that there are five water molecules for each unit of CuSO4.
2) How could you have calculated the mass of anhydrous salt and water using only the periodic table
and the initial mass of hydrate? (Note: These are the QUESTION(S) for this lab report. Your CLAIM for lab
A will solve these problems.)
Step 5. Write up your individual solution to the problems in step 4. When you have solved the problem in step
4, show your teacher your well-organized data and results and explain your written conclusions. Each
individual must get the teacher’s approval in writing before continuing with parts B or C. If you wish to avoid
the crowd in class, you should try to see your teacher outside of regularly scheduled class hours. You may not
go on to lab B or C until your teacher has approved your solution to part A. Write up a short lab report and
submit it by the assigned due date.
Step 6. Perform either hydrate lab B (magnesium sulfate, MgSO4 x H2O) or hydrate lab C (strontium
chloride, SrCl2 y H2O) as assigned by your teacher. Follow the general lab procedure outlined in step 1. Do
one trial for each group member.
Each of the hydrates is a salt with an unknown number of water molecules attached to each unit. Your
job is to determine the number of water molecules for each hydrate unit. Remember, hydrates come in whole
number ratios of one unit of salt to x or y number of water molecules. By performing the hydrate experiment,
and using your conclusion from part A, you should be able to determine the values of x or y in the formulas.
There are many methods of solution. You should use one that makes sense to you. Guess-and-check is NOT an
acceptable method of solution.
Step 7. Enter this data into the appropriate spreadsheet (B or C) (worth 10 pts. if you enter it by the deadline,
zero points if you are late).
Step 8. Write up a lab report for whichever lab you performed (B or C). The question you are answering with
your claim is “How many water molecules are present in each hydrate unit?” Clearly explain the reasoning
used in your evidence section.
Step 9. Turn in both short lab reports.
After the Lab A Conference, write up detailed notes of what you learned from the lab conference. Show
specific examples of each type of calculation with complete explanations as if you were writing a textbook. A
chemistry student from any high school in the area should be able to understand what you are talking about.
Grade breakdown:
Lab report A
100 pts
Lab A checkpoint with teacher
10 pts
Lab A conference conclusions
20 pts
Lab report B/C
100 pts
Evaluation Sheet
Hydrate Labs A, B & C
Safety
information
Data
Table
Claim
Evidence
Reading
Reflection
Writing
All chemicals involved and hazards identification. This
information can be obtained from an online MSDS.
(www.flinnsci.com is my favorite site for MSDS)
Create a data table for your lab report showing your data from lab.
Data needs to be recorded accurately and clearly, one trial for each
member of the group. Include all original masses (what the
balance said every time you used it) & the calculated mass of
hydrate, mass of anhydrous salt and mass of water.
Lack of attention to sig. figs will result in zero points for this entire
section.
Claim answers the question(s) in the lab. Warning: Lab A
contains 2 questions.
Claim is derived from the observations.
Claim is concise (one sentence per question – DO NOT EXPLAIN
YOURSELF HERE!).
Comes from observation.
Logically supports the claim with written explanations and sample
calculations.
Sample calculations are clearly labeled & easy to understand. If
you are not using YOUR data for the calculations, tell what data
you are using & why.
Results from all trials are in a clearly labeled, easy to understand
results table.
Lack of attention to sig. figs will result in serious point reduction!
Is your claim supported by the scientific literature? Other student
work is not considered to be scientific literature – even if it is
posted on the Internet.
Tell what your source is (give URL or page numbers of a text I
have access to), tell what it says and how it supports your claim &
evidence.
If calculating % error is a useful way to compare your answer to
the scientific literature then do those calculations.
How confident are you in your results?
If outside sources did not agree with your personal data, try to
identify what went wrong with your lab.
What would you do differently if you had to do this lab over?
Correct spelling and complete sentences.
Ideas are presented such that the reader can easily understand
them.
Entered data into the spreadsheet by the second (Lab A) or last (Lab B/C) day of lab:
5 pts
10 pts
10 pts
30 pts
10 pts
5 pts
20 pts
10 pts
___________
Total: 100 points