DENSITY OF MATTER

Objective
To identify an unknown metal based on density measurements. To determine the  density of  diluted rubbing alcohol.

Introduction
The density is one of the physical properties of matter. The density of a substance is defined  as the ratio of its  mass to its  volume:
                                                         d = m / V                                                Eq. 1

The usual units for density in the SI (International System of Units) are g/mL for liquids,  g/cm3 for solids, and g/L for gases. The density of a pure substance in its solid state is usually greater than its density in the liquid state. Water is an important exception-  ice floats on water. Gases are much less dense than solids or liquids.  The density of a substance is usually reported at a specific temperature, since its value is temperature dependent.
The density is determined indirectly,  based on  measurements of the  mass and volume of the substance.
The mass of a substance is measured using a balance. Analytical balances are very accurate instruments that measure mass to the nearest  0.0001 g. The top-loading balances that you will use in this lab will determine the mass to the nearest 0.01 g, which is adequate for this experiment.
The volume of liquids is determined  using volumetric glassware such as volumetric  pipets, graduated cylinders  or pycnometers. Measurements made with graduated cylinders  are not as precise  as those performed with volumetric pipets.

The volume of  regularly shaped objects (spheres, cubes, cylinders, etc.) can be determined by measuring the dimensions of the object and calculating the volume based on the appropriate mathematical equation. For example, if the object was a cube with the side length  a= 2 cm, the volume would be calculated as V= a3 = (2 cm)3 = 8 cm3.

Determining the  volume of   irregularly shaped solids (e.g. metal samples in this experiment), requires more ingenuity.  Archimedesí principle can be used to measure their volume. This principle states that an object, when placed in a liquid, will displace a  volume of liquid equal to its own volume. However, to be able to measure the volume of an object by displacement method, the object has to be denser than the liquid used and must be completely immersed in the liquid. The most commonly used liquid is water.

Procedure
I. Determining the Concentration of the Unknown Isopropyl (Rubbing) Alcohol Solution.
In this  part of the  experiment, you will  determine the concentration of an  unknown solution of rubbing (isopropyl) alcohol in water. The density of a rubbing alcohol/water mixture depends on the concentration of alcohol.  To find the relationship between density of the solution  and the alcohol concentration in this solution,  you need to  determine the density of several solutions with a known concentration of alcohol  first.

1. Obtain three isopropyl alcohol solutions in water with the following concentrations of alcohol (% weight): 20%,  50% and  70%.

2. Weigh a 10 mL graduated cylinder. The cylinder has to be CLEAN AND DRY. Use tongs or test tube holder to handle the cylinder.  Record the  tare (empty cylinder) mass on the data sheet.

3. Pour 5-6 mL of the first  isopropyl alcohol solution of known concentration into the tared cylinder, taking care not to spill any on the outside of the cylinder. Read and record the volume of the alcohol in the cylinder to the nearest 0.1 mL (or 0.2 ml, depending on the cylinder).

Note: it is convenient to perform the measurements starting with the lowest or highest concentration and continue in  increasing or decreasing order.

4. Weigh the cylinder with alcohol and record the weight.

5. Discard  the alcohol into the waste container. Clean and dry the cylinder.

6. Repeat steps 2 through 5 with the remaining 2 known alcohol solutions.

7. Obtain an unknown sample and repeat steps 2-5 using this sample. Record the weights and volume.
 

II. Identification of an Unknown Metal
Measure the volume of an irregular metal sample by displacement  method. When  you are  done, pour the water into the sink. Pour the metal sample on a paper towel. DRY the metal sample and return to the cart. Identify the metal  by comparing the experimentally obtained value with the values listed in Table 1.
Table 1.
 
Metal sample form  Amount  of metal used  Size of the graduated cylinder used Amount of water used in the cylinder
uniform silver-color pellets  10-15 pieces if the pieces will fit:  10 mL
if do not fit:              50 mL
   approx. 5 mL
irregular silver-color chunks  8 smaller pieces or 5 larger     50 mL    approx. 30 mL
orange/reddish granules/spheres  approx. 15 grams     10 mL    approx. 5 mL
tiny dark gray spheres  approx. 30 grams    10 mL    approx. 5 mL

Calculations
I. Rubbing Alcohol
1. Calculate the mass of each alcohol solution used by subtracting the mass of an empty cylinder from the mass of the cylinder with alcohol.
2. Calculate the density of each known  solution and the unknown using the formula in Eq. 1.
3. Determine the  approximate concentration of the unknown solution by  fitting the value of density obtained for the unknown into the set of  densities obtained for the three known solutions.
II. Unknown metal
1. Calculate the volume of  your unknown metal sample.
2. Calculate the  density of the unknown metal.
3. Identify the unknown metal by comparing the obtained density with the values in Table 2.

 Table 2.
Metal                            Density,  g/cm3                               Metal                            Density,  g/cm3
Aluminium                                     2.70                                                    Zinc                                               7.13
Copper                                          8.96                                                   Osmium                                      22.57
Lead                                            11.36                                                    Gold                                          19.31
Magnesium                                   1.74
 

Data Sheet
I. Rubbing Alcohol

Uknown #____________
Alcohol solution concentration,  % weight        Mass of empty 
          cylinder 
            grams 
Mass of cylinder with 
            alcohol,
             grams
Mass of alcohol
          solution,
            grams
Volume of alcohol 
            solution,
                  mL
          Density 
              g/mL
20
50
70
Unknown = 
Calculations (show one example calculation for each step  for the uknown) :
Mass of  unknown alcohol sample =

Density of  the unknown sample  =

The unknown rubbing alcohol solution #_______ was determined to have the
concentration_______________ beacuse______________________________________________________________________________
 

II. Uknown Metal
Unknown # __________               Description of  the metal sample__________________________________________________

Check Table 1 to find out how much metal to use and what size cylinder.
         Mass of
   weighing  dish, 
           grams
        Mass of  weighing dish and 
        sample,
           grams
   Mass of metal,
           grams
Volume of water in the cylinder, V1
            mL
Volume of water after sample was 
         added,  V2
            mL
     Metal  volume, 
            mL
Metal density, 
       g/cm3

 

 


 

 


 

 


 

 


 

 


 

 


 

 

Calculations:
Mass of metal  =
 

Volume of metal =
 

Density of metal =
 

The uknown metal sample #   _____    was identified as _______________ beacuse its experimentally determined density  ________g/cm3 closely matched the density  of that metal sample listed as ________g/cm3.
 

PRELAB
CLICK ON IT!

POSTLAB
1. Calculate the volume (in m3 ) of a 25,000 tonne iceberg. ( 1tonne = 1,000 kg)