Chem 4
WORKSHOP
1. Which of the following statements of the atomic theory proposed by John Dalton at
the beginning of the 19th century are not quite true in light of modern atomic
physics? Rewrite each statement to reflect the current understanding of the atomic
theory.
An element is made up of atoms. All
atoms of a given element are identical.
Atoms cannot be created or destroyed.
All atoms of a given element have the same
number of protons in the nucleus. They may
differ in the number of neutrons. Chemically the
atoms of a given element are virtually
indistinguishable: the types of chemical
reactions are the same; the rates may slightly
differ for different isotopes.
Atoms cannot be created or destroyed in
chemical reactions. In chemical reactions the
old bonds between atoms are broken down
and new bonds are formed. Atoms, however,
can be created or destroyed in nuclear
reactions: radioactive decays, nuclear fission
and fusion.
All atoms of one element have the mass. Atoms of different elements may
Atoms of two different elements have
combine in the ratio of small, whole
different masses.
numbers to form compounds.
The main characteristic of an element is the
atomic number. Two elements differ from each
other by their atomic numbers. Atoms of two
different elements may have the same mass
numbers and atomic masses that are very
close to each other.
This is often the case for many compounds,
especially inorganic. It was always the case for
compounds known to Dalton. Most organic
compounds (whose formulas were not known
to Dalton) do not follow that rule: C12H22O11
(sugar), C27H46O (cholesterol), C17H21NO4
(cocaine).
2. Use the density table to complete the following table.
Example
#
Element
Compound
Homogeneous
Mixture
Heterogeneous
Mixture
1
diamond
sugar
gasoline
cork
2
zinc
sodium chloride
olive oil
wood
3
mercury
ethyl alcohol
blood plasma
bone
4
helium
methane
cement
5
nitrogen
carbon dioxide
milk
3. Below is the picture showing the path a proton flying between the poles of a
magnet. On the same picture draw the path for a neutron and electron.
4. Manganese has only one naturally occurring isotope, manganese-55.
(a) What is the mass number of that isotope?
55
(b) What is the atomic number of that isotope?
25
(c) What is the mass in amu of a single atom of that isotope?
54.938049 amu
(d) Do all naturally occurring manganese
atoms contain the same number of protons?
yes
(e) Do all naturally occurring manganese
atoms contain the same number of neutrons?
yes
(f) What is the atomic weight of manganese (element)?
54.938049 amu
5. Below is a graph showing the relationship between mass and volume for water.
Within the same coordinate plane draw a graph showing the relationship between
mass and volume for aluminum.
6. Below is a cube that approximately corresponds to one gram of water. Next to it
draw (approximately to the scale) two cubes: a cube of one gram of gold and a cube
of one gram of air.
7. Determine the thickness, in mm, of a 7.0 cm × 3.5 cm piece of aluminum foil if it has
a mass of 0.1260 g. Do you need anything else to answer this problem? If so, look it
up.
Ans.: 1.9×10−2 mm
Volume = 7.0 cm × 3.5 cm × Thickness cm
Volume = Mass / Density = 0.1260 g / (2.70 g/cm3) = 0.04666 cm3
Thickness = Volume / (7.0 cm × 3.5 cm) = 0.04666 cm3 / (7.0 cm × 3.5 cm)
= 0.0019 cm = 0.019 mm
8. A quick method of determining density utilizes Archimedes' principle, which states
that the buoyant force on an immersed object is equal to the weight of the liquid
displaced. A bar of magnesium metal attached to a balance by a fine thread
weighed 31.13 g in air and 19.35 g when completely immersed in hexane (density
0.659 g/cm3). Calculate the density of this sample of magnesium.
Mass of magnesium = 31.13 g
Volume of magnesium = (31.13 g – 19.35 g) / (0.659 g/cm3) = 17.87 cm3
Density = Mass / Volume = 31.13 g / (17.87 cm3) = 1.74 g/cm3
(31.13 g – 19.35 g) is the buoyant force which is equal to the volume of hexane
displaced by the bar of magnesium.