Experiment #2 / Unit 3
Using the Chemical Counting Unit – Part II: Compounds
Introduction:
Due to the extremely small size of atoms and molecules, chemists must
have extremely large numbers of them in order to see and handle them.
Counting units are made up in order to simplify any task that requires working
with large numbers of objects. A "dozen" eggs and a "ream" of paper are
examples. The "chemist's dozen" is the mole. A mole of atoms refers to 6 x 10 23
atoms. Working with moles allows us to avoid dealing with large numbers all the
time. For example, if we had 1.2 x 1024 atoms of Cu, this would simplify to exactly
2 moles. However, we do not have a mole balance to measure this number
directly. We must determine the number of moles of a substance indirectly.
Usually this involves massing the substance and converting to moles using the
relative masses from the periodic table. We have already used the mole system
to “count” atoms. Now we will use the same system to work with compounds.
Procedure:
*The questions posed here will be answered when you carry out the
calculations on page 2.
A. How many formula units of calcium carbonate are in a piece of chalk?
1. Obtain a piece of chalk and mass it on the balance.
B. How many water molecules is a mouthful?
1. Obtain a 400 mL beaker and a 100 mL graduated cylinder.
2. One member of the team will fill their mouth with water by drinking from the
water fountain and then transfer the water into the graduated cylinder using
the beaker.
3. Measure the volume of the water.
C. How many sucrose molecules are in a sugar cube?
1. Obtain a sugar cube and mass it on the balance.
D. How many formula units of sodium chloride are in one “crystal” of table salt?
1. Count out and mass about (must know accurately) 20 “crystals” of table salt.
E. How many molecules of silicon dioxide are in a grain of sand?
1. Count out and mass about (must know accurately) 20 grains of sand.
Data:
 mass of chalk (A)




volume of water (B)
mass of sugar cube (C)
mass of salt crystals (D)
mass of sand grains (E)
Chemistry I
Cary Academy
W.G. Rushin
1
Calculations and Questions:
 Round all molar masses to the tenths place. Round your answers to the
correct number of sf. Do not determine sf from your molar masses nor
Avogadro's number. Treat them as exact numbers. Must solve using FLM.
Identify units and substance in all steps. Cancel units where appropriate.
 Estimates must show some method of scaling.
1. Estimate how many formula units of calcium carbonate (CaCO3) made up the
piece of chalk. Now calculate how many formula units of calcium carbonate
(CaCO3) made up the piece of chalk.
2. Calculate the mass of water in a mouthful.
3. Estimate how many water molecules were in a mouthful. Now calculate how
many water molecules were in the mouthful.
4. Estimate how many sucrose (C12H22O11) molecules make up the sugar cube.
Now calculate how many sucrose molecules make up the sugar cube.
5. Calculate the mass of one salt “crystal”.
6. How many formula units of sodium chloride (NaCl) are in one “crystal”.
7. Calculate the mass of one grain of sand.
8. How many molecules of silicon dioxide (SiO2) are in one grain of sand?
Lab Report #3.2:
 title page
 procedure sheet
 data table
 calculations and questions
Chemistry I
Cary Academy
W.G. Rushin
2