Density and % error

Density

• Density – Compares mass (amount of matter) of object to its volume (amount of space occupied)

• mass per unit volume (g/cm3)

D = MV D

M

V

Density

• Mass is always in g

• Volume is always in cm3

• If not… you must convert

• Note:

• I L = 1dm3

• 1mL = 1 cm3

g/ cm 3

Is considered to be a derived unit because it combines 2 SI units

Example

• Calculate the density of an object that has a volume of 4.67 cm3,and a mass of 56.9 g

• D = m V

• D= x• M= 56.9g• V= 4.67 cm3

Examples

• X = 56.9g

4.67 cm3

• x=12.2 g/cm3

• * RULE FOR SIGNIFICANT DIGITS WHEN MULTIPLYINMG OR

DIVIDING APPLY HERE

Density

• An object has a volume of 825 cm3 and a density of 13.6 g/cm3. Find its mass.

GIVEN:

V = 825 cm3

D = 13.6 g/cm3

M = ?

WORK:

M = DV

DM

V

Density

A liquid has a density of 0.87 g/mL. What volume is occupied by 25 g of the liquid?

GIVEN:

D = 0.87 g/mL

V = ?

M = 25 g

DM

V

WORK:

V = M D

Density

You have a sample with a mass of 620 g & a volume of 753 cm3. Find density.

GIVEN:

M = 620 g

V = 753 cm3

D = ?

DM

V

WORK:

D = M V

Ways to determine density

1. Regularly shaped objects – direct volume measurement (l x w x h)

2. Irregularly shaped objects- use water displacement method (graduated cylinder) to find volume.

Note: mass is found the same way regardless the shape of the object

Density

• I have a block that measures 5.25 cm by 2.25 cm by 8.50 cm.

• I weigh the block and find its mass to be 5.85 g

• Calculate the density of the block in g/cm3

For example

• A chunk of metal has a mass of 5.25 g. It is placed in a graduated cylinder containing 25.0 ml of water. Once the metal is placed in the graduated cylinder, the water rises to 38.2 ml. What is the density of the metal?

% error

• % error – the deviation from the accepted value during an experiment. Usually caused by human error.

• % error = (your value - literature value) x 100

• literature value

• It is always a positive number

Page 38 in textbook has a list of densities