Question

Density of plastic

Your challenge in this experiment is to determine the density of a mystery plastic, using two different step-by-step procedures. For sufficient error analysis, each method should be completed at least twice (e.g., Method A: Trial #1 & Trial #2; Method B: Trial #1 & Trial #2).
The plastic is in the form of small (~1 cm), irregularly shaped pieces. You will have access to several pieces of the plastic. The irregular shapes mean that it will not be possible to calculate the volume of the pieces by measuring their dimensions. In addition to the normal chemistry laboratory equipment and water, there will be methanol and ethylene glycol available in the lab this week.

My question is

Write two detailed, step-by-step procedures that use different ways to determine the density of the plastic.

Answer 1

1)The density of something is just the mass divided by the volume:

D = m/V

The mass you can measure on a balance or a scale, and the volume is the amount of space the object occupies. You can find the volume of an irregular object by immersing it in water in a beaker or other container with volume markings, and by seeing how much the level goes up.

2)As described in the previous question the density is given by D = M/V. If you don't know V then you have to compare the density of the unknown to that of a known density, e.g. water.
Call the density of the object D_o and the density of water D_water.   The weight of the object in air is then M_air = Do* V. The weight of the object in water is M_water = (D_o - D_water) * V.
Dividing one equation by the other the volume V cancels out and the ratio of the two measurements is M_air/M_{water =}  D_o/(D_o - D_water). Using the value of D_water as 1 gram per cubic centimeter you can easily solve this equation for the density of the object D_o