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

2. Lesson 75 & 76: Make It Count
Counting By Weighing & Avagadro’s #

3. ChemCatalyst The LD50 of arsenic (III) oxide, As2O3, is 15 mg/kg.
Figure out the lethal dose for a 150 lb adult.
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?

4. Key Question
How can mass help you count large numbers of small objects?

5. You will be able to:
explain how large numbers of small objects are determined
calculate the percent error of a calculation
Express and interpret numbers in scientific notation

6. 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.

7. Discussion Notes (cont.)
Chemists use mass when measuring chemical compounds because it is not possible to count atoms directly.
Chemists use percent error to express how close their measurements are to the accepted value.
Percent error = |observed value – actual value| • actual value

8. 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.
Move the decimal point four places to the left.
1.56 X 104 = 15,600
1.56 X 10–4 =

9. 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.

10. Wrap Up - Continued 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.

11. 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?
If you have 1 mol of aluminum and 1 mol of iron, which has more mass? How many atoms are in each sample?