SCIENTIFIC MEASUREMENT

To calculate Density, you have to know how to measure volume and mass properly and accurately!

Reporting Measurements

• Using significant figures

• Report what is known with certainty

• Add ONE digit of uncertainty (estimation)

Davis, Metcalfe, Williams, Castka, Modern Chemistry, 1999, page 46

The instrument determines the amount of precision of the data.

• What is the certain measurement here? • What is the estimated measurement here?

Using Significant Figures reflects precision by estimating the last digit

• What is the certain measurement? • What is the estimated measurement?

Measurements in the Lab:

Beaker marked in 10 mL increments.

The volume you write in your lab report should

be 13 mL

Buret marked in 0.1 mL increments. You record

volume as 0.67 mL

1. If the glassware is marked every 10 mLs, as is this beaker, the volume you record should be in whole mLs. (Example A)

2. If the glassware is marked every 1 mL, as is this graduated cylinder, the volume you record should be in tenths of mLs. (Example B)

3. If the glassware is marked every 0.1 mL, as is this buret, the volume you record should be in hundredths of mLs. (Example C)

20 mL

30 mL

10 mL

0 mL

1 mL

2 mL

Example AExample B

40 mL

30 mL

20 mL

Example C

Graduated Cylinder

marked in 1.0 mL

increments. you record volume as

35.5 mL

Practice Measuring

4.5 cm

4.54 cm

3.0 cm

Timberlake, Chemistry 7th Edition, page 7

cm0 1 2 3 4 5

cm0 1 2 3 4 5

cm0 1 2 3 4 5

Finding VOLUME

Finding VOLUMEWhat is VOLUME?

The amount of space that a 3-dimensional object or substance

takes up. Anything that exists is made of matter

and therefore has volume…(takes up

space)

The OBJECTS

The OBJECTS

Formula/Equation?

Length = 6cm

Volume = L xW x H

Finding Volume

Length = 6cm

Height = 4cm

Volume = L x W x H

Finding Volume

Length = 6cm

Height = 4cmWidth = 2cm

Volume = L x W x H

Finding VolumeVolume = L x W x H

Length = 6cm

Height = 4cmWidth = 2cm

6 cm x 2 cm x 4 cm =

48 cm3

Volume = L x W x H =

So, the VOLUME (the amount of space that this 3 dimensional

object takes up) is 48 cm3

The OBJECTS

Find the VOLUME

The METHOD – Finding Volume by Water

Displacement

-40mL

-35mL

-30mL

-25mL

-20mL

-15mL

-10mL

-5mL

1. Note the water level

-40mL

-35mL

-30mL

-25mL

-20mL

-15mL

-10mL

-5mL

2. Place the object in liquid

-40mL

-35mL

-30mL

-25mL

-20mL

-15mL

-10mL

-5mL

-40mL

-35mL

-30mL

-25mL

-20mL

-15mL

-10mL

-5mL

3. Watch the object displace the water it is place into…

4. Note difference in water level

-40mL

-35mL

-30mL

-25mL

-20mL

-15mL

-10mL

-5mL

The water level changed 15mL

We can say the rock displaced 15mL of water

-40mL

-35mL

-30mL

-25mL

-20mL

-15mL

-10mL

-5mL

35 mL-20 mL

15 mL

But wait, it gets better!

-40mL

-35mL

-30mL

-25mL

-20mL

-15mL

-10mL

-5mL

Volume of our rock…

Density is defined as mass per unit volume. It is a measure of how tightly packed and how heavy the atoms are in an object. Density is the ratio of mass to volume

You can think of Density as the amount of stuff per unit of space

How close the atoms or molecules are to each other.

More than “heaviness” - density includes how much space an object takes up!!

All substances have density including liquids, solids, and gases.

We need to be able to think about density visually, logically and

mathematically

Density is the measure of the “compactness” of a material

Visually - Which one is more dense?

• Demonstration: Atoms in a substance• Which square is more dense? circle one

Masses are / are not equalVolumes are / are not equal

Visually - Which one is more dense?

• Now… which one is more dense?

Masses are / are not equalVolumes are / are not equal

Logically - What would take up more space? A kilogram of

feathers…..or a kilogram of steel?

OR

MathematicallyDETERMINING DENSITY

Regular Shapes – mass, then determine the volume by formula/equation EX: cubes, rectangular prisms,

cylinders, spheres, cones, etc. Irregular shapes – mass, then measure

displacement of a liquid (usually water) by that irregularly shaped object

Since we know that 1 mL has the same volume as 1 cm3, we can make an easy conversion!

Density = mass (g) volume (cm3 or mL)

Equation to Calculate a Substance’s DENSITY

For our terms and purposes:Mass is usually expressed in grams

Volume usually expressed in cubic centimeters (cm3)

or milliliters (mL)Density may be expressed in other units, but you will learn about them in Physics and Chem….

DENSITY - an important and useful (qualitative) intensive

physical property

Density mass (g)volume (cm3)

Density mass (g)volume (cm3)

Mercury

13.6 g/cm3 21.5 g/cm3

Aluminum

2.7 g/cm3

Platinum

Density Calculations of:

1. Regularly Shaped Solids:mass = triple beam or electronic

balancevolume = measure (l x w x h)

2. Irregularly Shaped Solids:mass = triple beam or electronic

balancevolume = water displacement

Density Calculations of:

3. Liquids: mass = tare graduated cylinder,

mass liquid volume = read graduated cylinder

We will perform labs on all of these types of density calculations!

Osmium is a very dense metal. What is its density in g/cm3 if 50.00 g of the metal occupies a volume of 2.22 cm3?

1) 2.25 g/cm3

2) 22.5 g/cm3

3) 111 g/cm3

Learning Check – Mathematical Calculations of Density

Placing the mass and volume of the osmium metal into the density setup, we obtain

D = mass = 50.00 g = volume 2.22 cm3

= 22.522522 g/cm3 = 22.5 g/cm3

Learning Check – Mathematical Calculations of Density

Which diagram represents the correct order of liquid layers in the cylinder?(K) Karo syrup (1.4 g/mL), (V) vegetable oil (0.91 g/mL,) (W) water (1.0 g/mL)

1) 2) 3)

K

K

W

W

W

V

V

K

Learning Check – Mathematical Calculations of Density

V

The density of octane, a component of gasoline, is 0.702 g/mL. What is the mass, in kg, of 875 mL of octane?

1) 0.614 kg

2) 614 kg

3) 1.25 kg

Learning Check – Mathematical Calculations of Density

Learning Check – Mathematical Calculations of Density

If blood has a density of 1.05 g/mL, how many liters of blood are donated if 575 g of blood are given?

1) 0.548 L2) 1.25 L3) 1.83 L

4) 548 L

Which is heavier, a quart of water or a quart of mercury?

If we have equal volumes of two different substances, then the one with the greater

density will have the greater mass.

Fact: Water has a density of 1.0 g/ml and mercury (Hg) has a density of 13.6 g/ml.

Learning Check – Logical Thinking about Density

►Which is heavier, a ton of feathers or a ton of bricks?

►Which is larger?

►If two objects have the same mass, the one with the higher density will be smaller.

Neither

Feathers

Factors Affecting DensityWrite the relationship statements.

Temperature –

What substance is the exception to this rule?

Pressure –

Factors Affecting DensityA solid solutes dissolved into liquid solutions

– the density change depends on the concentration and kind of substances mixed together.

Write the relationship statement:

Factors Affecting DensityAtomic mass – Different atoms have different atomic

masses. Write the relationship statement.

Applying What You’ve Learned – Let’s think about the Density of Water!

Question #1: At what temperature is water most dense?

Find this information on your ESRT’s! Hint: Look for a section about the Properties of Water…

Question #2: Use this chart to explain why ice floats on water. Use data from the chart to support your answer!!!!

Question #3: Use this chart to explain what happens to average sea level when the ocean’s average temperature increases.

Density TableSINK or FLOAT

In Water?(D = 1.0 g/mL)

Float

Float

Float

Sink

Sink

Sink

Float

Float

(alcohol)

(fuel)

Specific GravityThe density of a material or substance,

relative to another substanceExpressed in a ratio: water = 1.0 g/cm3

Water is the substance to which we compare other substances

Also known as SPECIFIC GRAVITY Since we dived the density of any

substance by 1.0 g/cm3, the specific gravity value is equal to the density of the substance.

What are the units for specific gravity?