LECTURE 2
TITLE: STIOCHOIMETRY AND CALCULATIONS
Date: August 26, 2003
Outline:
1. Units and Composition
2. Stiochoimetric Calculations
3. In Class Exercises
1. Units and Composition
The composition of a sample should be immediately understandable and
usable. This is complicated by history and common usage. Thus,
composition can be either simple or complex
Simple: a direct link between the substance in a sample and the amount of
the substance.
Gases in Air:
Gas
Dry air %
By volume
By weight
Oxygen
Nitrogen
Carbon dioxide
Hydrogen
Argon
Neon
Helium
Krypton
Xenon
20,99
78,03
0,03
0,00005
0,93
0,0018
0,0005
0,0001
9 10-6
23,20
75,47
0,046
~0
1,28
0,0012
0,00007
0,0003
0,00004
The composition of air is unchanged until the height of approximately 10.000 m.
Sugar in blood…mg/dl
Time of Check
Goal plasma blood glucose
ranges
for the non-diabetic
Goal plasma blood glucose
ranges
for people with diabetes
Before Breakfast (fasting)
< 110
90 - 130
Before lunch, supper and snack
< 110
90 - 130
Two hours after meals
< 140
< 160
Bedtime
< 120
110 - 150
A1C (also called Glycosylated
hemoglobin A1c, HbA1c or
glycohemoglobin A1c)
< 6%
< 7%
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People from outside the US may find this table convenient for converting US blood
glucose values which are given in mg/dl into values generated by their blood glucose
meters, which are generated in mmol/L.
mmol/L mg/dl
mmol/L mg/dl
mmol/L mg/dl
0.06
1
6.7
120
16.0
288
0.28
5
7.0
126
16.6
300
0.55
10
7.2
130
17.0
306
1.0
18
7.5
135
18.0
325
http://www.joslin.harvard.edu/education/library/wbggoal.shtml
Sodium in drinking water- < 20 mg/L
http://www.oznet.ksu.edu/library/H20QL2/MF1094.PDF
Red Dye No. 40 in twizzlers: in ppm
FDC DYE CONTENT OF FOODS
CATEGORY
AVERAGE CONCENTRATION (PPM)
CANDY AND CONFECTIONS
100
BEVERAGES (LIQUID AND
POWDERED)
75
DESSERT POWDERS
140
CEREALS
350
MARACHINO CHERRIES
200
PET FOODS
200
BAKERY GOODS
50
ICE CREAM AND SHERBETS
30
SAUSAGE (SURFACE)
125
SNACK FOODS
200
http://class.fst.ohio-state.edu/fst621/Additive%20classes/color/color.htm
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Semi-simple: Others may be stiochoimetric equivalents but not really the
chemical form.
Pb in gasoline = In 1982, leaded gasoline contained 1.25 gram/gal (&
accounted for 86% of the lead in the atmosphere). See:
http://www.uwsp.edu/geo/courses/geog100/Lead-Science.htm
details about tetra ethyl lead
http://www.osha.gov/dts/chemicalsampling/data/CH_270900.html
Iron in Blood
Ferritin
Men:
18–270 nanograms per milliliter (ng/mL)
Women:
18–160 ng/mL
Children:
7–140 ng/mL
Infants (up to 5 months old):
25–200 ng/mL
Ferritin is a protein in the body that binds to iron. Most of the iron stored in the
body is attached to ferritin.
Ferritin is found in the liver , spleen , and bone marrow. Only a small
amount is found in the blood. The amount of ferritin in the blood may help
indicate the amount of iron stored in your body.
This test is most commonly done on a blood sample taken from a vein. Less
commonly, the amount of iron in the body can be evaluated during a bone
marrow analysis.
http://my.webmd.com/hw/womens_conditions/hw6212.asp
Mercury in Fish (actually methyl or dimethyl mercury)
http://www.epa.gov/ost/fishadvice/mercupd.pdf
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Complex Composition
Still other may have historic references and are distantly related to actual
chemical…elemental composition or a derivative. Others may be specified in
terms of the measurement (optical units).
Composition of gold- Karats Fineness or Karats of Gold
The proportion of gold in jewellery is measured on the carat (or karat) scale. The word carat comes from
the carob seed, which was originally used to balance scales in Oriental bazaars. Pure gold is designated
24 carat, which compares with the "fineness" by which bar gold is defined. A Carat (Karat in USA &
Germany) was originally a unit of mass (weight) based on the Carob seed or bean used by ancient
merchants in the Middle East. The Carob seed is from the Carob or locust bean tree. The carat is still used
as such for the weight of gem stones (1 carat is about 200 mg). The earliest known gold jewellery dates
from the Sumer civilisation, which inhabited what is now southern Iraq around 3000 BC. Articles
displaying various techniques such as repousse, chain- making, alloying and casting have been found in
ancient Egyptian tombs, with the best known examples coming from the treasures of King Tutankhamun
who died in 1352 BC. The Minoans on Crete produced the first known cable chain, still very popular
today, and the Etruscans in Italy had developed granulation, whereby items are decorated with tiny
granules of gold, by the 7th century BC.
Purity of Gold
Percent
Fineness
Karats
Part of Au per 100 Parts of Au X/24 parts
per 1000
per 1000
100%
1000 fine
24 K
91.67
916.7 fine
22 K
75.0
750 fine
18 K
58.3
583.3 fine
14
41.67
416.7 fine
10
See http://www.gold.org/discover/knowledge/aboutgold/gold_jewellery/index.html
The most widely used alloys for jewellery in Europe are 18 and 14 carat, although 9 carat is popular in
Britain. Portugal has a unique designation of 19.2 carats. In the United States 14 carat predominates, with
some 10 carat. In the Middle East, India and South East Asia, jewellery is traditionally 22 carat (sometimes
even 23 carat). In China, Hong Kong and some other parts of Asia, "chuk kam" or pure gold jewellery of 990
fineness (almost 24 carat) is popular
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Seeds of the carob tree (Ceratonia siliqua) were once used as a standard for weighing precious gemstones. An
average carob seed weighs about 3 1/3 grains, just over 200 milligrams. [Note: Some references give 4 grains for a
carob seed.] The brass weight in photo is exactly one gram (1000 milligrams), or about 5 carob seeds. The "carat" is
our present-day, international unit for the weight of diamonds. Carat is derived from the word "carob," in reference to the
carob seed. One carat is precisely 200 milligrams. The diamond in the engagement ring shown above is 1.09 carats or
218 milligrams. Karat spelled with a "k" refers to the fineness or purity of gold. One karat is 1/24 or 4.2 percent pure gold
in an alloy. 16-karot gold is 16/24 or 66.7 percent pure gold, while 24-karat gold is 24/24 or 100 percent pure gold.
Fertilizers are often listed as NPK values
Source of nutrients % Nitrogen (N) % Phosphoric % Potassium (K2O) %
Acid (P2O5)
Water
Ammonium nitrate
33.0
0
0
Ammonium sulphate
21.0
0
0
Calcium nitrate
15.5
0
0
Potassium chloride
Potassium nitrate
0
13.0
0
0
62
44
Potassium sulphate
0
0
50
Superphosphate
0
20
0
Triple superphosphate
0
46
0
This is not done to frustrate and confuse…often arises from human
intransigence or convenience or history.
The units for composition can be quite varied and include these as common:
weight percent: fertilizer,
volume percent: gasoline
fractional composition or ratios (parts per million very common): humidity,
concentrations (moles/liter): medical and medicinal
enzyme, units???
Conversions are possible between unit systems…in some instances we need
to know stiochoimetry. This may seem a little abstract at present….but will
become central to our efforts in Chem 371.
2. Stiochoimetric Calculations
The ratios of elements in a formula or the molar coefficients all fit under my
broad definition of Stiochoimetric Calculations
For a review of Stoichiometry see:
http://dbhs.wvusd.k12.ca.us/Stoichiometry/Stoichiometry.html
“The word stoichiometry derives from two Greek words: stoicheion (meaning "element") and
metron (meaning "measure"). Stoichiometry deals with calculations about the masses (sometimes
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volumes) of reactants and products involved in a chemical reaction. It is a very mathematical part
of chemistry, so be prepared for lots of calculator use.
Jeremias Benjaim Richter (1762-1807) was the first to lay down the principles of stoichiometry. In
1792 he wrote:
"Die stöchyometrie (Stöchyometria) ist die Wissenschaft die quantitativen oder Massenverhältnisse
zu messen, in welchen die chymischen Elemente gegen einander stehen." [Stoichiometry is the
science of measuring the quantitative proportions or mass ratios in which chemical elements stand
to one another.]”
Another good review site:
http://www.shodor.org/UNChem/basic/stoic/
Stoichiometry is involved in percent composition and in calculations of mass
of elements from formulas: We can explore the mass relations of Ca in
Calcium oxalate monohydrate.
If I have 100 g of CaC2O4•H2O, how many grams of Ca do I have?
MM Ca
Grams Ca = Grams of CaC2O4•H2O x --------------------1
MM CaC2O4•H2O
That ratio
MM Ca
--------------------- =
MM CaC2O4•H2O
40.08
---------------- =
146.115
0.27431
2
This is the gravimetric factor:
grams Ca = 0.27431 * grams of calcium oxalate.
Answer: 27.43 grams
This is also seen in practical life with fertilizers (a good topic for Aggies).
Fertilizers are sold on basis of composition and nitrogen is a key ingredient.
Nitrogen is specified as %mass of N but naturally elemental nitrogen in not in
the fertilizer. How to make calculations? Stiochoimetric factors are like
gravimetric factors except no precipitate. Example is nitrogen in Urea
CO(NH4)2. What is the nitrogen content in urea?
Grams N
=
Grams CO(NH4)2
That ratio is shown here:
2*(MM N)
28.013
--------------------- =
---------------- =
MM CO(NH2)2
60.03
x
2*(MM N)
--------------------MM CO(NH4)2
0.46665
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4
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Urea has 46.7% nitrogen content and is thus a fairly impressive chemical.
FYI. Urea [(NH2)2CO] is prepared by reacting ammonia with carbon dioxide:
2 NH3 (g) + CO2 (g) Æ (NH2)2CO (aq) + H2O (l)
This shows 2 to 1 Stiochoimetry in the reactants.
Example in problem form: Percent phosphorus in a sample (see Day
and Underwood, 5th Ed, p 85).
The phosphorus in a sample of phosphate rock weighing 0.5428 g is
precipitated as MgNH4PO4*6H2O and ignited to Mg2P2O7. If the ignited
precipitate weights 0.2234 g, calculate:
A) percent P2O5 in the sample, and
B) the percent purity expressed as P rather than P2O5.
% P2O5
Grams P2O5
-------------------Weight of sample
=
Grams P2O5
=
Wt of Mg2P2O7MM P2O5
---------------------------------------- x100
MM Mg2P2O7
5
6
Since
MM P2O5 = 141.95; MM Mg2P2O7 = 222.55
Percent P2O5
Percent P
=
=
26.25
Wt of Mg2P2O7
-------------------Weight of sample
7
2 x MM P
---------------------x100
MM Mg2P2O7
8
MM P = 30.974; MM Mg2P2O7 = 222.55
Percent P
=
11.46
9
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You will remember Stiochoimetry in balancing equations and conversions
with moles. The number counting for atoms also appears in reaction
quantities. I have a helpful routine:
1. Sugar fermentation is described by the following reaction:
C6H12O6(aq) --------> C2H5OH(aq) + CO2(g)
What mass of C2H5OH is produced when 250.0 grams of C6H12O6 is formed from the
fermentation?
Balance the Reaction
C6H12O6(aq) -------->
C2H5OH(aq) +
CO2(g)
PRINCIPLES
Moles (Theory)
Molar Mass (g/mole)
ACTUAL STUFF
Mass (grams)
MOLECULAR STUFF
Moles (Actual)
Moles (Predicted)
2. What mass of HCl in needed to react with235.8 grams of Al(OH)3 according to the following
reaction?
Balance the Reaction
HCl
+
Al(OH)3
-------->
AlCl3 +
H2O
PRINCIPLES
Moles (Theory)
Molar Mass (g/mole)
ACTUAL STUFF
Mass (grams)
MOLECULAR STUFF
Moles (Actual)
Moles (Predicted)
3. In Class Assignment
Go to Seat work
Version 3.0 G.A. Eiceman, August 2004
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