10. A freshman chemist analyzed a sample of copper(II) sulfate pentahydrate for water of

hydration by weighing the hydrate, heating it to convert it to anhydrous copper(II)

sulfate, and then weighing the anhydride. The % H 2 O was determined to be 30%. The

theoretical value is 33%. Which of the following choices is definitely NOT the cause of

the error?

A. After the student weighed the hydrate, a piece of rust fell from the tongs into the

crucible.

B. Moisture driven from the hydrate condensed on the inside of the crucible cover before

the student weighed the anhydride.

C. All the weighings were made on a balance that was high by 10%.

D. The original sample contained some anhydrous copper(II) sulfate.

E. The original sample was wet.

Answer: E

30% H 2 O in the hydrate sample represents

mass of hydrate %

mass of hydrate mass of anhydride

# 100

In a problem like this, I like to make up some easy fictitious numbers that I can use to fit the
scenarios and see how the various changes affect the final outcome. Let’s say the mass of the
hydrate is 10 g and the mass of the anhydride is 7 g. This would translate as

g %%

gg

10 HO

10 7

(^100302)

• =

In examining choice A, the original mass of the hydrate would not change; however, because
rust will not evaporate, the final mass of the anhydride would be higher than expected — let’s
say 8 g. Substituting this value into the formula for % water would give

g %%

gg

10 HO

10 8

(^100202)

• =

which is less than the theoretical value of 30%. This is in the direction of the student’s experi-
mental results, and since we are looking for the choice that is NOT the cause, we can rule out A
as an answer.

In choice B, the mass of the hydrate would not change, but the mass of the anhydride would be
higher than it should be. Let’s estimate the anhydride at 8 g again.

g %%

gg

10 HO

10 8

(^100202)

• =

Part II: Specific Topics