Periodic Trends in Periodic Trends in DensityDensity

Required Laboratory SkillsRequired Laboratory Skills

Purpose of the labPurpose of the lab

KnowledgeKnowledge To describe how the density of elements To describe how the density of elements

changes with position on the periodic tablechanges with position on the periodic table

SkillsSkills To measure and record properties with the To measure and record properties with the

correct number of significant figurescorrect number of significant figures To round calculated values to the correct To round calculated values to the correct

number of significant figuresnumber of significant figures

How do you find the density of a How do you find the density of a handful of gravel?handful of gravel?

You need to measure TWO things!!You need to measure TWO things!! Mass and Volume…what do you use to Mass and Volume…what do you use to

measure these?measure these? Triple Beam BalanceTriple Beam Balance Graduated Cylinder (we’ll use the large ones today)Graduated Cylinder (we’ll use the large ones today)

Some considerationsSome considerations Gravel has an irregular shape so you’ll need to Gravel has an irregular shape so you’ll need to

put it in a container!put it in a container! On which measurement will we need to On which measurement will we need to

subtract the measurement of the container?subtract the measurement of the container? Only the mass!Only the mass!

Some thoughts…Some thoughts…

Is it okay to measure the mass of a Is it okay to measure the mass of a sample of the gravel while your sample of the gravel while your partner measures the volume of partner measures the volume of another sample of gravel?another sample of gravel?

Is it okay to measure the mass of a Is it okay to measure the mass of a sample of gravel when it is wet sample of gravel when it is wet because another group didn’t dry it because another group didn’t dry it off?off?

Proper Recording of MassProper Recording of Mass

373.34 grams

http://staff.tuhsd.k12.az.us/gfoster/standard/labeq3.htm

Proper Recording of VolumeProper Recording of Volume

52.7 mL

http://inst.sfcc.edu/chemscape/catofp/measurea/volume/gradcyl/gradcyl.htm

Notice…Notice…

The measurement for mass is more The measurement for mass is more preciseprecise than that for volume. than that for volume.

But…that is what we have to work But…that is what we have to work with, so we have to consider that with, so we have to consider that when performing our calculationswhen performing our calculations

MassMass

Let’s say the container weighs 2.43 g Let’s say the container weighs 2.43 g and the container with gravel weighs and the container with gravel weighs 15.32 g.15.32 g.

Since these are in the same unit and Since these are in the same unit and have the same precision, the answer have the same precision, the answer will have the same number of will have the same number of decimal places as the decimal places as the measurement… 12.89 gmeasurement… 12.89 g

VolumeVolume

Let’s say you put some water in the Let’s say you put some water in the graduated cylinder and record the graduated cylinder and record the volume as 30.7 mL. After you put volume as 30.7 mL. After you put the gravel in the volume is measured the gravel in the volume is measured as 32.6 mL.as 32.6 mL.

Since these are in the same unit and Since these are in the same unit and have the same precision, the answer have the same precision, the answer will have the same number of will have the same number of decimal places…1.9 mL.decimal places…1.9 mL.

DensityDensity

The mass was 12.89 g and the The mass was 12.89 g and the volume was 1.9 mLvolume was 1.9 mL

Since these are different units and Since these are different units and have different precision, we can no have different precision, we can no longer use the number of decimal longer use the number of decimal places to compare them!places to compare them!

DensityDensity

The mass was 12.89 g and the volume The mass was 12.89 g and the volume was 1.9 mLwas 1.9 mL

So we can only use the number of total So we can only use the number of total digits in the measurement to specify digits in the measurement to specify the precision.the precision.

The answer can only have as many The answer can only have as many digits as the measurement with the digits as the measurement with the leastleast digits! digits!

DensityDensity

The mass was 12.89 g and the The mass was 12.89 g and the volume was 1.9 mLvolume was 1.9 mL

Density = mass/volumeDensity = mass/volume Density = 12.89g / 1.9mLDensity = 12.89g / 1.9mL Density = 6.784210526 g/mLDensity = 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 So it should be rounded to … 6.8

g/mLg/mL