LIVING BY CHEMISTRYUnit 4: TOXINSStoichiometry, Solution Chemistry, and Acids and Bases

In this unit you will learn:• how toxins are defined• how chemists determine toxicity• the mechanisms by which toxic substances act

in our bodies and what this has to do with chemical reactions

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

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• 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

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• 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

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 mediumrubber 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

band0.57 g

small sticky note 0.15 g business card 0.80 g

#8 lock washer 0.136 gsmall metal paper

clip0.94 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

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• 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.

1.56 X 104 = 15,600

1.56 X 10–4 = 0.000156

Move the decimal point four places to the right.

Move the decimal point four places to the left.

Discussion Notes (cont.)

• Very small amounts of a substance contain enormously large numbers of atoms.

• The mole is simply a counting unit.

• The mass of 1 mol of a substance is called the molar mass.

1 mole = Avogadro’s number

= 602 sextillion

= 602,000,000,000,000,000,000,000

= 6.02 X 1023

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

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• 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

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

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

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• 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 cans64g / 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.