Name __________________________
Empirical Formula of a Hydrate Pre-lab
On a separate sheet of paper, please answer all questions in complete sentences, incorporating the questions.
For calculations, please show ALL WORK, and clearly label every number with a unit and a heading.
1. Define a hydrate and an anhydrate. You can find the definitions in your textbook.
2. What is one way described in the text to convert a hydrate to an anhydrate?
3. Is the conversion of a hydrate to an anhydrate a physical or a chemical change? Explain why you think
your answer is correct? Again, you can look up the definitions of a physical and chemical change in
your textbook.
4. In the lab you will be completing you will be taking a hydrate and heating it gently to try to evaporate
the water. As you evaporate the water, what should happen to the mass of your sample?
5. After heating it for several minutes, you decide all of the water has been evaporated so you cool your
sample and weigh it. If all of the water in the hydrate were evaporated after that first heating and you
were to then gently heat it a second time, what should happen to the mass of your sample?
6. If, after the first heating all of the water had not been evaporated, and you then heated the sample a
second time and evaporated even more water, what should happen to the mass after the second heating?
7. Describe a procedure which will ensure that all of the water has been removed from a hydrated
compound.
8. Quite often when a dehydrated hydrate is left to cool in a container which is open to the air the
dehydrated hydrate will reabsorb moisture from the air (especially when the air is very humid or very
wet like after a rain). Define a “dessicator” and a “dessicant”. How would they be beneficial in a lab
like this one?
9. Quite often, when a hydrated compound is heated and the water is evaporated, the compound will
gradually change color as it becomes an anhydrate. Describe a procedure for showing that this color
change was only a physical change, and not a chemical change.
10. The measurements and observations included in the following data table were made as blue solid copper
(II) Chloride was gently heated over a Bunsen burner.
Mass of empty crucible
25.760 g
Mass of crucible and copper (II) chloride
26.725 g
Mass of crucible and copper (II) chloride after first heating 26.525 g
blue solid
white solid with a whitish gas
coming out
Mass of crucible and copper (II) chloride after second heating
26.261 g
white solid
Mass of crucible and copper (II) chloride after third heating 26.261 g
white solid
After several drops of water were added to the crucible after the 3rd heating the white solid turned into a blue
solid and heat was given off.
a. What are two pieces of evidence from that data table that suggest that something was removed from the
zinc chloride?
b. What are two pieces of evidence that suggest that what was removed might have been water?
c. Why did the experimenter need to heat the sample a second time?
d. Why did the experimenter need to heat the sample a third time?
e. How did the experimenter know that they didn’t need to heat the sample a fourth time?
f. Give a possible explanation for why we might expect heat to be given off when water is added to an
anhydrate.
g. Using the data in the table above, show work for calculating the total amount of water that was removed
from the copper (II) chloride.
h. Using your answer to question “g” and a periodic table, show all work for calculating the moles of water
that was removed from this hydrated copper (II) chloride.
i. Using your data table, show all work for calculating the grams of anhydrous copper (II) chloride that was
left after all of the water was removed.
j. Using your answer to letters “i” and a periodic table, show all work for calculating the moles of copper
(II) chloride in your original sample.
k. Write the empirical formula of the hydrated copper (II) chloride.
l. Use your textbook to determine the name of this hydrated compound.