1. Periodic Trends in Density Required Laboratory Skills

2. Purpose of the lab Knowledge Knowledge To describe how the density of elements changes with position on the periodic table To describe how the density of elements changes with position on the periodic table Skills Skills To measure and record properties with the correct number of significant figures To measure and record properties with the correct number of significant figures To round calculated values to the correct number of significant figures To round calculated values to the correct number of significant figures

3. How do you find the density of a handful of gravel? You need to measure TWO things!! You need to measure TWO things!! Mass and Volume…what do you use to measure these? Mass and Volume…what do you use to measure these? Triple Beam Balance Triple Beam Balance Graduated Cylinder (we’ll use the large ones today) Graduated Cylinder (we’ll use the large ones today) Some considerations Some considerations Gravel has an irregular shape so you’ll need to put it in a container! Gravel has an irregular shape so you’ll need to put it in a container! On which measurement will we need to subtract the measurement of the container? On which measurement will we need to subtract the measurement of the container? Only the mass! Only the mass!

4. Some thoughts… Is it okay to measure the mass of a sample of the gravel while your partner measures the volume of another sample of gravel? Is it okay to measure the mass of a sample of the gravel while your partner measures the volume of another sample of gravel? Is it okay to measure the mass of a sample of gravel when it is wet because another group didn’t dry it off? Is it okay to measure the mass of a sample of gravel when it is wet because another group didn’t dry it off?

5. Proper Recording of Mass 373.34 grams http://staff.tuhsd.k12.az.us/gfoster/standard/labeq3.htm

6. Proper Recording of Volume 52.7 mL http://inst.sfcc.edu/chemscape/catofp/measurea/volume/gradcyl/gradcyl.htm

7. Notice… The measurement for mass is more precise than that for volume. The measurement for mass is more precise than that for volume. But…that is what we have to work with, so we have to consider that when performing our calculations But…that is what we have to work with, so we have to consider that when performing our calculations

8. Mass Let’s say the container weighs 2.43 g and the container with gravel weighs 15.32 g. Let’s say the container weighs 2.43 g and the container with gravel weighs 15.32 g. Since these are in the same unit and have the same precision, the answer will have the same number of decimal places as the measurement… 12.89 g Since these are in the same unit and have the same precision, the answer will have the same number of decimal places as the measurement… 12.89 g

9. Volume Let’s say you put some water in the graduated cylinder and record the volume as 30.7 mL. After you put the gravel in the volume is measured as 32.6 mL. Let’s say you put some water in the graduated cylinder and record the volume as 30.7 mL. After you put the gravel in the volume is measured as 32.6 mL. Since these are in the same unit and have the same precision, the answer will have the same number of decimal places…1.9 mL. Since these are in the same unit and have the same precision, the answer will have the same number of decimal places…1.9 mL.

10. Density The mass was 12.89 g and the volume was 1.9 mL The mass was 12.89 g and the volume was 1.9 mL Since these are different units and have different precision, we can no longer use the number of decimal places to compare them! Since these are different units and have different precision, we can no longer use the number of decimal places to compare them!

11. Density The mass was 12.89 g and the volume was 1.9 mL The mass was 12.89 g and the volume was 1.9 mL So we can only use the number of total digits in the measurement to specify the precision. So we can only use the number of total digits in the measurement to specify the precision. The answer can only have as many digits as the measurement with the least digits! The answer can only have as many digits as the measurement with the least digits!

12. Density The mass was 12.89 g and the volume was 1.9 mL The mass was 12.89 g and the volume was 1.9 mL Density = mass/volume Density = mass/volume Density = 12.89g / 1.9mL Density = 12.89g / 1.9mL Density = 6.784210526 g/mL Density = 6.784210526 g/mL But that’s REALLY precise… and a lie! But that’s REALLY precise… and a lie! So it should be rounded to … 6.8 g/mL So it should be rounded to … 6.8 g/mL