The Percent of Water and Formula of a Hydrated Crystal
A paper submitted to the Lone Oak Chemistry Omnibus
11/11/09
KM
I. Title: The Percent of Water and Formula of a Hydrated Crystal
II. Date: 11/11/09
III. Objective: The purpose of this experiment was to determine the percent of water and the
formula of a hydrated crystal.
IV. Procedure and Data:
Procedure
1. An empty evaporating dish was massed,
then was massed again with 4 blue crystals.
Data
•Mass evaporating dish = 30.87 g
•Mass evaporating dish and hydrated crystals
= 32.63 g
•Substance = Copper II Sulfate Pentahydrate
2. The dish with the 4 blue crystals was
heated on a ring stand over a bunsen burner
until the crystals powdered.
3. After the dish cooled, the dish was massed
again.
•Mass evaporating dish and dehydrated
crystals = 32 g
V. Analysis:
1. Mass hydrated crystals = Mass evaporating dish and hydrated crystals - mass evaporating dish
Mass hydrated crystals = 32.63 g - 30.87 g
Mass hydrated crystals= 1.76 g
2. Mass dehydrated crystals = Mass evaporating dish and dehydrated crystals - mass evaporating
dish
Mass dehydrated crystals = 32 g - 30.87 g
Mass dehydrated crystals = 1.13 g
3. Mass water = Mass evaporating dish and hydrated crystals - mass evaporating dish and
dehydrated crystals
Mass water = 32.63 g - 32 g
Mass water = .63 g
4. % Water = Mass part | 100
Mass whole |
= .63 g H2O
| 100
1.76 g CuSO4 ⦁ 5H2O |
= 35.7% Water
5. Moles CuSO4 = Mass dehydrated crystals (CuSO4) | 1 mol CuSO4
|Atomic mass CuSO4
= 1.13 g CuSO4 | 1 mol CuSO4
|160 g CuSO4
= .007 mol CuSO4
6. Moles H2O = Mass water | 1 mol H2O
| Atomic mass H2O
= .63 g H2O | 1 mol H2O
| 18 g H2O
= .035 mol H2O
7. Mole ratio and formula =
CuSO4
H2O
.007/.007
=
CuSO4
⦁
.035/.007
5H2O
8. Theoretical % Water = Mass part | 100
Mass whole |
= 90 g H2O
| 100
250 g CuSO4 ⦁ 5H2O |
= 36% Water
9. % Water % Error
= Experimental - Theoretical | 100
Theoretical
= 35.7% - 36% | 100
36%
= -.83% Error
VI. Conclusions:
A. If the mass of the water in a hydrate (H2O) and the mass of the whole hydrate (CuSO4 ⦁
5H2O) can be found, then both the percent of water and the formula of the hydrated crystal
can be determined.
B. If the formula calculated for the hydrate was incorrect because the ratio for water was low
then the substance was probably not completely dehydrated. The mass for the dehydrated
substance was too large and the amount of water too small leading to an incorrect mole ratio
for the copper II hydrate pentahydrate.
C. In this experiment, copper II sulfate pentahydrate was dehydrated using a heat source. The
resulting anhydrous copper II sulfate, combined with other data, was used to determine the
percent of water in copper II sulfate pentahydrate. The empirical formula also was calculated
to be CuSO4 ⦁ 5H2O. By using the mass of hydrated and dehydrated crystals and the mass of
the evaporating dish, the empirical formula of the hydrate and percent of water can be
determined.