Density Determinations

NAME:______Period:______
Objective:

  • to determine the density of several solid materials using the formula for area (regularly shaped object) and water displacement (irregularly shaped object)
  • to determine the density of water and an unknown liquid and to compare these densities to the accepted value for each liquid using the percent error formula
  • to create a graph of mass vs volume for four data points and to calculate slope for that graph – to understand that the slope calculation represents the density for that substance

Introduction:

In this lab you will be determining the densities of several materials:

(1) water

(2) another liquid

(3) A regularly shaped solid

(4) An irregularly shaped solid

(5) Aluminum pellets

We will also be using a graphical method for density determination if time allows. Density is the ratio of an object’s mass to its volume. Density is usually expressed in units of g/mL or g/cm3. Note that the common medical term “cc” refers to “cubic centimeter” and is essentially the same as a mL. Technically, any units of volume and mass can be used. As you make your measurements, pay close attention to using the proper number of significant figures in your final answers.

Materials:

  • Water
  • A second unknown liquid sample
  • A regularly shaped solid
  • An irregularly shaped solid (rock)
  • Metal pellets
  • Dropper pipets
  • A balance
  • A ruler
  • Three graduated cylinders; 10 mL, 25 mL and 100 mL

Procedure:

PART 1: Liquids

**You will do this part TWICE – once for water then for your unknown liquid

  1. Weigh an empty dropper pipet on the balance.
  2. Fill the dropper with water and reweigh it to obtain the mass of the water alone.
  3. Obtain the weight of the water by subtraction.
  4. Squirt all of the liquid into the small (10 mL) graduated cylinder and measure the volume of the water.
  5. Calculate the density of the water (this is the “experimental” value).
  6. Calculate your percent error by obtaining the accepted (correct) value for the density of water and plugging in to the error equation: experiemental – accepted / accepted X 100
  7. Repeat steps 1 through 5 for you unknown liquid.

ACCEPTED VALUE FOR THE DENSITY OF WATER: ______

ACCEPTED VALUE FOR THE DENSITY OF ISOPROPOL ALCOHOL: ______

DATA TABLE A: Density of liquids

WATER / UNKNOWN LIQUID
Mass of empty pipet (g)
Mass of full pipet (g)
Mass of liquid (g)
Volume of liquid (mL)
CALCULATOR ANSWER / SIG FIG ANSWER / CALCULATOR ANSWER / SIG FIG
ANSWER
DENSITY CALCULATION (g/mL)
SHOW WORK HERE!
PERCENT ERROR
SHOW WORK HERE!

PART 1: Regularly shaped solid (block)

  1. Determine the mass of the object.
  2. Measure the length, width, and height in cm and calculate the volume (area).
  3. Calculate the density of the block.

DATA TABLE B: Density of a regularly shaped solid (block)

Mass of block (g)
Length x width x height (in cm)
CALCULATOR ANSWER / SIG FIG ANSWER
DENSITY CALCULATION (g/cm3)
SHOW WORK HERE!

PART 1: Irregularly shaped solid (rock)

  1. Determine the mass of the rock.
  2. Place about 50 mL of water in the large (100 mL) graduated cylinder and then read the meniscus to obtain an accurate reading of the volume.
  3. Gently place the rock in the water and re-measure the volume.
  4. Determine the volume of the rock by subtraction (this is known as volume by displacement).
  5. Calculate the density of the rock.

DATA TABLE C: Density of irregularly shaped solid (rock)

Mass of rock (g)
Volume of water BEFORE displacement (mL)
Volume of water AFTER displacement (mL)
Volume of rock (subtract) (mL)
CALCULATOR ANSWER / SIG FIG ANSWER
DENSITY CALCULATION (g/mL)
SHOW WORK HERE!

PART 2: Metal pellets

  1. Fill a 25 mL graduated cylinder to about the 10 or 12 mL mark. Record the volume carefully and accurately.

Initial volume in graduated cylinder:______

  1. Weigh two pellets and record their mass.
  2. Gently add them to the graduate and re-measure the volume. Obtain the volume of the pellets by displacement (subtract).
  3. Weigh two more pellets and record their mass. Add this mass to the mass of the two pellets in step 2 and record this mass in your data table as the mass of 4 pellets.
  4. Gently add them to the graduate and record the volume. Obtain the volume of these TWO pellets by displacement then add this value to the volume of the pellets in step 3 to obtain the volume of 4 pellets.
  5. Repeat these steps for 6 and 8 pellets.
  6. Calculate the density (mass/volume) for each of your trials and record these values in the data table. You should have four values for density and they should all be about the same number.
  7. Obtain the accepted (correct) value for the density of the metal pellets from your teacher and calculate your percent error using the average of your four densities as your experimental value.
  8. Create a graph of mass vs volume. Volume in mL should be on the X-axis and mass in grams should be on the Y-axis. The slope of this line should represent the density of aluminum. Plug your numbers into the slope equation (rise/run) and see if you were correct! Paste your graph into your lab report.

DATA TABLE D: Density by displacement – Metal Pellets

Pellets / Mass (grams) / Volume (mL) / Density(g/mL)
2
4
6
8

Average density from above:______

Accepted value for density of aluminum (what you should get):______

Experimental value for density (what you got):______

Percent error calculation (SHOW WORK HERE!):______

Post lab questions:

  1. When determining the volume of an object by displacement, should you determine the mass of the object before or after you place it in the water? Why?

______

______

  1. Why does a helium balloon rise in the atmosphere?

______

  1. Why does ice float in water?

______

  1. A sample of metal has a mass of 15.2 grams and a volume of 4.76 mL. What is the density of the metal? Think about how many significant figures your answer should have.

______

______

  1. What is wrong if a student reports the density of an object as 1.42857 g/mL if the object’s mass was 10.0 g and its volume was 7.0 mL?

______

______

  1. If the density of water is 1.00 g/mL what is the mass of 50 mL of water?

______