1. Living By Chemistry SECOND EDITION
Unit 4: TOXINS
Stoichiometry, Solution Chemistry, and Acids and Bases

2. Lesson 74: Lethal Dose
Toxicity

3. 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.

4. Key Question
How much is too much of a substance?

5. You will be able to:
calculate the toxic dose for a variety of substances, given the LD50
explain the role of dosage in toxicity

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.