Living By Chemistry
Unit 4: TOXINS
Stoichiometry, Solution Chemistry, and Acids and Bases
Section II: Weighing Toxins
Lesson 7 Lethal Dose
Lesson 8 Make It Count
Lesson 9 Billions and Billions
Lesson 10 What’s in a Mole?
Lesson 11 Mountains into Molehills
Lesson 12 How Sweet It Is
Lesson 7: Lethal Dose
Toxicity
ChemCatalyst
Which substance do you think is most toxic to
you—alcohol (ethanol, C2H6O), aspirin (salicylic
acid, C7H6O3), or arsenic (III) oxide (As2O3)?
Explain your thinking.
Key Question
How much is too much of a substance?
You will be able to:
• calculate the toxic dose for a variety of
substances, given the LD50
• explain the role of dosage in toxicity
Prepare for the Classwork
You will work individually or in pairs.
Lethal dose (LD50): the amount of an ingested
substance that kills 50% of a test sample of
animals. It is expressed in mg/kg, or milligrams of
substance per kilogram of body weight.
Discussion Notes
Toxicity depends on two quantities: amount of
toxic substance and mass of the organism.
All substances are toxic in large enough doses,
even sugar and water.
Discussion Notes (cont.)
The smaller the lethal dose, or LD50, the more
potentially dangerous a substance is.
Many toxic substances can be therapeutic at lower
doses.
Wrap Up
How much is too much of a substance?
• Toxicity is relative to dosage: The toxicity (or
therapeutic effect) of a substance depends on
the dose in which it is received.
• The lethal dose (or therapeutic dose) of a
substance is often expressed as a ratio between
a certain mass of the substance and 1 kilogram
of the body mass of an organism exposed to the
substance.
Check-in
Methadone is a medication used as a painkiller and
as a treatment for those recovering from heroin
addiction. The LD50 for methadone is 95 mg/kg.
a. Would you consider methadone to be more or less
toxic than acetaminophen (LD50 = 2404 mg/kg)?
than aspirin (LD50 = 200mg/kg)?
b. Explain how you would calculate the amount of
this substance that would be lethal to a 120 lb
human.
Lesson 8: Make It Count
Counting By Weighing
ChemCatalyst
The LD50 of arsenic (III) oxide, As2O3, is 15 mg/kg.
a) Figure out the lethal dose for a 150 lb adult.
b) How many atoms do you think are in a lethal
dose of arsenic (III) As2O3? What would you
need to know in order to find out?
Key Question
How can mass help you count large numbers of
small objects?
You will be able to:
• explain how large numbers of small objects are
determined
• calculate the percent error of a calculation
Prepare for the Activity
Work in groups of four.
Discussion Notes
The easiest way to determine the count of very
small objects is to find their total mass and divide
by the mass of one object.
In order to get a more accurate average mass
measurement for a tiny object, it is better to find
the mass of 10 or 20 of the objects and divide by
the number of objects to find the average mass,
especially if the variation in size is slight.
Discussion Notes (cont.)
Chemists use mass when measuring
chemical compounds because it is not
possible to count atoms directly.
Discussion Notes (cont.)
Chemists use percent error to express how close
their measurements are to the accepted value.
Percent error = |observed value – actual value| • 100
actual value
Discussion Notes (cont.)
Object
Mass (g)
Object
Mass (g)
seed bead
0.0056 g
large pony bead
0.26 g
sequin
0.0085 g
#18 medium
rubber band
0.27 g
grain of rice
0.022 g
elbow pasta
0.287 g
tiny rubber band
0.048 g
large sticky note
0.40 g
small pony bead
0.067 g
plastic paper clip
0.45 g
split pea
0.090 g
kidney bean
0.55 g
#10 small rubber band
0.126 g
#33 large rubber
band
0.57 g
small sticky note
0.15 g
business card
0.80 g
#8 lock washer
0.136 g
small metal paper
clip
0.94 g
Wrap Up
How can mass help you count large numbers of
small objects?
• It is possible to count large numbers of small
objects by weighing them together and dividing
by the mass of a single object.
• It is more accurate to weigh a large sample of a
collection of objects and find their average mass
than to rely on weighing a single object.
• There are so many atoms in a sample that you
cannot count them. Chemists use mass to
calculate numbers of atoms.
Check-in
You have a sandwich bag containing raisins. It
weighs 24.6 g. A sample of ten raisins weighs
0.90 g. The empty bag has a mass of 2.90 g.
How many raisins are in your sandwich bag?
Lesson 9: Billions and Billions
Avogadro’s Number
ChemCatalyst
Which do you think is more toxic, 1 mol of
arsenic, As, or 10 g of arsenic? Explain your
reasoning.
Key Question
What is the relationship between mass and moles?
You will be able to:
• translate numbers into scientific notation, and
vice versa
• explain the magnitude of a mole
• define molar mass for an element and find its
value on the periodic table
Prepare for the Classwork
Work individually or in pairs.
1 mole ≈ 602,000,000,000,000,000,000,000,
or 602 sextillion
Discussion Notes
Scientific notation is a convenient way to
write numbers that have lots of zeros, either
because they are very large or because
they are very small.
Discussion Notes (cont.)
A number in standard notation can be converted to
scientific notation by writing it as a decimal with one
digit to the left of the decimal point times a power of
ten.
Move the decimal point
four places to the right.
1.56 X 104 =
15,600
Move the decimal point
four places to the left.
1.56 X 10–4 = 0.000156
Discussion Notes (cont.)
Very small amounts of a substance contain
enormously large numbers of atoms.
The mole is simply a counting unit.
1 mole = Avogadro’s number
= 602 sextillion
= 602,000,000,000,000,000,000,000
= 6.02 X 1023
The mass of 1 mol of a substance is called the
molar mass.
Wrap Up
What is the relationship between mass and moles?
• One mole of a substance is equal to 602
sextillion—or 602,000,000,000,000,000,000,000
—objects. This is also called Avogadro’s number.
• Scientific notation is a convenient way to express
numbers that have many zeros.
• The atomic mass given on the periodic table is
equivalent to the mass of 1 mole of atoms of the
element in grams, called molar mass.
• Molar mass allows you to convert between moles
of atoms and grams of atoms.
Check-in
If you have 1 mol of aluminum and 1 mol of iron,
which has more mass? How many atoms are in
each sample?
Lesson 10: What’s in a Mole?
Molar Mass
ChemCatalyst
Consider 12 nickels, 2 empty aluminum cans,
and a balloon full of carbon dioxide gas.
a. Which has the greatest mass?
b. Which has the greatest number of atoms?
c. Which has the greatest number of moles of
atoms? Explain the reasoning behind your
answers.
Key Question
How can you convert between mass and moles?
You will be able to:
• calculate the molar mass of compounds
• describe the approximate magnitude of a mole
of a substance
• complete simple conversions between mass
and moles
Prepare for the Lab
Work in groups of four.
Discussion Notes
You can figure out the mass of 1 mole of any
element or compound using a periodic table.
A mole of atoms or molecules of a solid or a liquid
is an amount you usually can hold in your hand.
A mole of any gas, if it is at standard temperature
and pressure, always has a volume of 22.4 L.
Wrap Up
How can you convert between mass and moles?
• The molar mass of a compound is the sum of the
atomic masses of the atoms in the compound.
• The molar mass of a compound allows you to
convert between moles of the compound and
grams of the compound.
Wrap Up (cont.)
• One mole of a solid or a liquid is an amount that
you usually can hold in your hand.
• One mole of a gas occupies a volume of 22.4 L.
This is larger than the volume of 1 mol of a liquid
or a solid because of the space between gas
molecules.
Check-in
You have 1 mol of oxygen molecules, O2, and 1
mol of carbon dioxide molecules, CO2. Which has
more mass? Which has a larger volume at room
temperature?
Lesson 11: Mountains into
Molehills
Mass-Mole Conversions
ChemCatalyst
Arsenic, As, arsenic (III) oxide, As2O3, and
arsenic (III) sulfide, As2S3, are all toxic because
they contain arsenic.
a. Which is more toxic, 1 mol of As or 1 mol of
As2O3? Explain your thinking.
b. Which is more toxic, 1 g of As2O3 or 1 g of
As2S3? Explain.
Key Question
How are moles related to mass?
You will be able to:
• convert the number of moles of a compound or
an element to mass in grams
• convert the mass of a sample in grams to moles
Prepare for the Classwork
Work in pairs.
Discussion Notes
The relationship between mass (g) of a substance
and number of moles is proportional.
Mass (g) = k • moles
The proportionality constant, k, is equal to the
molar mass of the substance.
Discussion Notes (cont.)
The relationship between the number of particles of
a substance and the number of moles is also
proportional.
Number of particles = k • number of moles
In this case, the proportionality constant, k, is equal
to Avogadro’s number, 6.02 X 1023.
Wrap Up
How are moles related to mass?
• In order to convert moles to mass,
multiply the number of moles by the molar
mass.
• In order to convert mass to moles, divide
the number of grams by the molar mass.
Check-in
A sample of chlorine gas, Cl2, has a mass of 11 g.
How many moles of Cl2(g) is this?
Lesson 12: How Sweet It Is
Comparing Amounts
ChemCatalyst
Consider two cans of carbonated soft drink. One
is regular, and the other is diet.
a. Which do you think is more toxic, the regular
soft drink or the diet soft drink? Explain your
reasoning
b. What information would help you answer this
question?
Key Question
How can you use moles to compare toxicity?
You will be able to:
• use moles and molar mass to compare the
amounts of different substances
• discuss the safety of sweeteners
Prepare for the Demonstration
Work individually.
Discussion Notes
Aspartame molecules are much sweeter than
fructose molecules.
The safety of artificial sweeteners has been a
subject of debate for years.
Discussion Notes (cont.)
It is interesting to compare the LD50 for fructose
and for aspartame.
Regular soft drink
(fructose)
Diet soft drink
(aspartame)
Lethal dose for
a 64 kg person
29,7000 mg/kg • 64 kg =
1,900,000 mg = 1,900 g
10,000 mg/kg • 64 kg
= 64,000 mg = 64 g
Number of cans
1,900 g/40 g/can ≈ 48 cans
64g / 0.225 g/can ≈
280 cans
Wrap Up
How can you use moles to compare toxicity?
• The smaller the LD50, the more toxic a
substance is. However, the LD50 values are of
limited use in determining the long-term
effects of a substance.
Check-in
Imagine two substances of equal mass with
molecules that are equally toxic.
Would you rather be exposed to the substance with
the larger molar mass or the smaller molar mass?
Explain your reasoning.