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

2. Lesson 75: Make It Count Counting By Weighing

3. ChemCatalyst The LD 50 of arsenic (III) oxide, As 2 O 3, 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) As 2 O 3 ? 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

6. Prepare for the Activity Work in groups of four.

7. 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.

8. Discussion Notes (cont.) Chemists use mass when measuring chemical compounds because it is not possible to count atoms directly.

9. 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

10. Discussion Notes (cont.) ObjectMass (g)ObjectMass (g) seed bead0.0056 glarge pony bead0.26 g sequin0.0085 g #18 medium rubber band 0.27 g grain of rice0.022 gelbow pasta0.287 g tiny rubber band0.048 glarge sticky note0.40 g small pony bead0.067 gplastic paper clip0.45 g split pea0.090 gkidney bean0.55 g #10 small rubber band0.126 g #33 large rubber band 0.57 g small sticky note0.15 gbusiness card0.80 g #8 lock washer0.136 g small metal paper clip 0.94 g

11. 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.

12. 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?