CH 110 MSUB Avogadros Number Calculations Questions

Rev. 9/2017AVOGADRO’S NUMBER
INTRODUCTION1
Avogadro was honored for his contributions to science by the use of his name with a
number that has been accepted as a standard in chemistry. In a paper published in
1811, Avogadro hypothesized that equal volumes of gases under conditions of equal
temperature and pressure contain equal numbers of molecules. Gay-Lussac had
found that gases reacting chemically under conditions of equal temperature and
pressure combine in ratios of small whole numbers. Although many were puzzled
by the apparently strange results of Gay-Lussac’s experiments, Avogadro’s solution
to this problem was rejected by the majority at that time. Neglected for about fifty
years, Avogadro’s hypothesis was revived by Cannizzaro. It was accepted then
because evolving ideas on the nature of gases yielded powerful evidence in this
favor. This phase of the development of ideas in chemistry has been termed “the
final stage of the chemical revolution of the eighteenth and nineteenth centuries; the
result was the development and sharpening of many of the fundamental concepts
upon which modern chemistry is built”.2
The acceptance of Avogadro’s hypothesis enabled the determination of the relative
weights of many atoms and molecules; atomic weights and molecular weights were
then defined in terms of the accepted standard, 0 = 16.0000 amu. What is the
present standard? The gram atomic weight, or gram atom, and the gram molecular
weight, or mole, were defined as the standard “package” of atoms or molecules.
The name “Avogadro’s number” was given to the number of molecules in 1 mole and
the number of atoms in 1 gram atom. The number 6.02 x 10 23 is now referred to as
a “mole” in the same manner that the number 144 refers to a gross.
Avogadro’s number has been found by various methods, all of which yield the same
results, within the limits of experimental error. The method we shall use today,
although primitive, yields surprisingly good results which are of the right order of
magnitude if the experiment is carried out carefully. It utilizes an interesting property
of certain large molecules such as fatty acids. If a drop of fatty acid is placed on the
surface of water, it will spread out to form an extremely thin film. Observations of
this sort were recorded as long ago 1773 by Benjamin Franklin who noted that 1
teaspoon of oil spread out to form a film of about 22,000 square feet on a pond near
London. Franklin was investigating an old observation – the effect of pouring oil on
troubled waters.
1
Reference for additional discussion: L. Carrol King and E. K. Neilson, J. Chem. Educ., 35, 198 (1958).
2
G. Holton & D. Roller, Foundations of Modern Physical Science, Addison-Wesley Publishing Co., Reading, Mass., 1958, p. 400.
15
– Avogadro’s Number –
The thickness of this extremely thin film is related to one of the dimensions of the
long-chain molecules in the film. This can be demonstrated by placing a wire across
the surface of a shallow container filled to the brim with water, and allowing a drop of
oil to fall on the water to one side of the wire. The oil will spread out over the water
surface and attach itself to the wire and to the edges of the container because of
intermolecular forces. If the wire is moved so as to compress the film and
sufficiently reduce the area occupied by the film, then the film buckles. This occurs
because the molecules in the film cannot be pushed together indefinitely since each
molecule in the film has a cross sectional area associated with it. When the
molecules are packed together as tightly as possible and further force is applied to
compress the film, the film buckles and the molecules slide over each other.
In this experiment, we will use the long chain fatty acid called oleic acid, C 18H34O2,
for generating the film on the surface of H2O. Fatty acids in general contain both a
polar, hydrophilic end and a non-polar hydrophobic part. The hydrophilic part of the
molecule e.g. -COOH in fatty acids, is attracted to H2O. The non-polar part of the
molecule is not attracted to H2O. It is now recognized that the fatty acid molecules
form a film one molecule thick (mono-layer). The molecules have a vertical or near
vertical orientation in the film, like cigarettes in a pack.
This interesting behavior of large insoluble molecules is related to the following: (1)
the attractive forces between the polar end of the fatty acid molecule and the H 2O,
(2) the intermolecular forces of attraction between the remainder of the chains,
which are non-polar.
The thickness of the film, which is the length of one molecule, may be calculated
from a knowledge of the volume of oleic acid in the drop comprising the film, and the
area of the film, since volume of film = (area) x (thickness). If the simplifying
assumption is made that the molecules are boxes with a length 10 times larger than
their width, then the volume of one (box-shaped) molecule is equal to (width) x
(width) x (length). From the known density and molar mass of oleic acid, the molar
volume (volume of one mole) is calculated, using the definition of density (d = m / v)
Finally, Avogadro’s number can be determined by dividing the molar volume by the
molecular volume to determine the number of molecules in one mole.
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– Avogadro’s Number –
PROCEDURE
In this experiment the importance of immaculately clean equipment (especially
the cake pan) cannot be over-emphasized. A small amount of grease from your
finger will result in unreliable values. Clean (with glassware soap and distilled
water) a cake pan, then dry with paper towels.
From the front table, obtain a 9″ disposable pasteur pipet. Remove the rubber
bulb from the standard eye dropper in your drawer and attach it to the pasteur
pipet. The volume of a single drop from this dropper needs to be determined. To
do this, pour approximately 2 mL of the oleic acid solution into a 10 mL
graduated cylinder and record the initial volume (+0.01 mL). Pour some more
oleic acid solution ( ~ 2 mL more) into a small test tube. Use this supply in the
test tube and the pasteur pipet to add exactly 100 drops of the oleic acid solution
into the graduated cylinder. Record the final volume (+0.01 mL).
Add distilled water to the previously cleaned cake pan to a depth of a few
centimeters. Evenly dust the surface with a thin layer of lycopodium powder.
Too much powder will cause the oil film to buckle, whereas too little powder will
cause the film to separate. The lycopodium powder makes the boundaries of the
oil film easily visible.
Fill your dropper with some oleic acid from the graduated cylinder or test tube,
discard the first drop (which is usually a partial drop or a bubble). Then put 1 full
drop of solution on the surface of the water near the center of the pan. The
alcohol dissolves, leaving a layer of oleic acid.
When the motion of the powder has ceased, with a wax pencil, trace the shape of
the open area of the film onto a glass plate placed carefully on top of the cake
pan. Remove the glass plate and transfer the area traced to graph paper.
Determine the area of the film by counting the squares within the outline or by
cutting and weighing the paper within the outline and comparing to the mass of a
known area of paper.
Complete the calculations on the following page and enter your data and
calculations into the laboratory computers to receive a print-out, be sure to have
your instructor sign the print-out as well as all of your laboratory notebook pages.
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– Avogadro’s Number –
CALCULATIONS ** DON’T FORGET LABELS, UNITS AND SIGNIFICANT FIGURES !
1.
volume of 1 drop = (volume added) =
(Vf – Vi )
100
2.
volume of oleic acid in 1 drop (Note: the solution is 0.50% oleic acid )
= volume of 1 drop x (0.0050)
3.
molar mass of oleic acid (C18H34O2) (report answer with 3 sig. figs.)
4.
density oleic acid = 0.894 g/cm3 (use as a given value)
5.
molar volume (cm3 / mole for oleic acid) remember: density = mass
Volume
6.
area of film (report value + 1 cm2)
(be sure to show work for the method used)
7.
thickness of film
(note: film was created by the volume of oleic acid in one drop) also 1 mL = 1 cm
3
volume of film = (area) x (thickness)
8.
length of a molecule (report answer with 2 s.f.)
length of one molecule is assumed to be the same as the thickness of the
film if the molecules are standing straight up
9.
width of a molecule (report answer with 2 s.f.)
10.
width = length
10
volume of 1 molecule, assuming molecules are boxes as pictured below (2 s.f.)
molecule volume = (width) x (width) x (length).
11.
number of molecules in 1 mole (2 s.f.)
=
molar volume
molecule volume
18
CHEMICAL STOICHIOMETRY PRE-LAB – Fall 2020
Name
Lecture prof.
This exercise is due at the beginning of your regular lab period during the week of 2/10/2020.
This assignment is worth 15 points.
BRING TO LAB – CHEMISTRY TEXT BOOK & LECTURE NOTES & A SCIENTIFIC CALCULATOR.
You will NOT be allowed to use your phone as a calculator.
1.
Complete the table:
Symbol
Element name
# of protons
# of neutrons
40
2+
20𝐶𝑎
32 216𝑆
56
3+
26𝐹𝑒
14
7𝑁
127 153𝐼
2. Name the following compounds:
NaBr
Al2O3
Fe(NO3)2
KSCN
Co(OH)2
3. Write the chemical formula for the following compounds:
copper (I) carbonate
iron(III) sulfate
potassium permanganate
barium phosphate
ammonium chromate
# of electrons
Net charge