5 Steps to a 5 AP Chemistry

282  Step 4. Review the Knowledge You Need to Score High

Measurements

1. the mass of the crucible and cover


2. the mass of the original sample and the crucible and cover


3. the mass of the reacted sample and the crucible and cover

The last measurement must be done after the sample has cooled to room temperature.

Calculations

The mass of the sample is calculated by taking the difference between masses 1 and 2. The

mass of the substance that combined with the original substance is calculated from the

difference between masses 2 and 3. The moles of the substances must be calculated by

dividing each mass by the molar mass of the substance. The empirical formula is calculated

from the simplest ratio of the moles of the elements present.

Comments

This procedure will allow you to calculate the empirical formula of the substance. The

experiment is often performed by reacting magnesium metal with atmospheric oxygen to

form magnesium oxide.

Experiment 2: The Percentage of Water in a Hydrate

Synopsis

The amount of water present in a sample is determined by weighing a compound before and

after heating. The difference in mass is due to the loss of water. (See the chapter on

Stoichiometry.)

Equipment

analytical balance burner crucible and cover

support stand test tube(s) tongs

triangle crucible support wire gauze

Measurements

1. the mass of the crucible and cover


2. the mass of the original sample and the crucible and cover


3. the mass of the heated (dried) sample and the crucible and cover

The last measurement must be done after the sample has cooled to room temperature.

Calculations

The mass of the hydrate is calculated from the difference between masses 1 and 2. The mass

of the water lost is calculated from the difference between masses 2 and 3. The percentage

of water is calculated by dividing the mass of the water lost by the mass of the hydrate and

multiplying the result by 100%.

A variation of this experiment uses the mass of the anhydrous material (calculated from

the difference between masses 1 and 3). The moles of the anhydrous material and water are

then calculated from their respective masses and molar masses. The simplest ratio of the

moles gives the empirical formula.