This is the eighth in a series of 8 chemistry labs on the fundamentals of biodiesel.
In this lab we will learn about “Laboratory 8: Density Measurement of Chemicals and Fuels”.
Written procedures and background information for all 8 labs is provided on the biodiesel
education website.
The density of a substance is the mass per of an object divided by its volume.
Density is a way of quantifying how heavy or light a substance is for a given amount
of volume.
For example, if 1 liter of vegetable oil weighs 930 grams, the density of the oil is 930 grams
divided by one to equal 930 grams per liter.
The mass of a substance is the sum of the masses of all individual molecules.
Therefore, the density of a substance can be changed by changing the number of molecules
occupying a unit volume.
For instance, when a substance is heated, the heat energy increases the molecular vibration,
pushing tightly packed neighboring molecules farther apart.
As a result, a given number of molecules occupies more volume without increasing its mass.
Therefore, when volume increases without increasing the mass, the density decreases.
When two substances are mixed together, several things could happen.
Let’s imagine mixing substance A with substance B.
If the intermolecular distance of substance A is large enough that substance B can snugly
fit into the void, the density of the mixture will be higher than that of substance A.
This also holds true when a chemical compound is dissolved.
For example, quite a lot of salt can dissolve in water without increasing the water volume.
The process increases the density of the chemical mixture.
This is the reason salt water is about 2.5% heavier than fresh water.
In the second type of mixture, the voids are not big enough for substance A to accommodate
substance B. In this case the mixture volume is the sum of the volume of the individual
components.
An example of this is the mixture of atmospheric gases.
In the third type of mixture, component A chemically reacts with component B and the
product of the reaction may have lower or higher density, depending on the chemical
reaction.
The density of the product depends on the chemical structure of the molecule formed.
The density of matter in the gaseous phase can be calculated mathematically from its
molecular formula.
One mole of gas always occupies 22.4 liters at standard temperature and pressure.
Therefore, the density of a gas at standard temperature and pressure can be calculated
as shown here.
However, the density of liquids and solids is not as straightforward as that of gases.
In this lab we will begin by calculating the mass of methanol needed, and how much biodiesel
and glycerol are produced, from 100 mL of soybean oil reacted.
The biodiesel transesterification is given by this chemical reaction.
From previous discussions we know that the density of vegetable oil is about 930 grams
per liter.
From the transesterification chemical reaction, we can simply divide three times the molecular
weight of methanol by molecular weight of vegetable oil and multiply that by 930.
Similarly, the mass of biodiesel and glycerol produced can be calculated as shown here.
Now, take the temperatures of the soybean oil, biodiesel and petroleum diesel and ensure
that they are all at room temperature.
If not wait until they all come to room temperature.
Pour the 100 grams of vegetable oil in a measuring cylinder using a dropper and a weighing scale.
Then pour calculated mass of biodiesel produced from 100 grams of vegetable oil in a separate
measuring cylinder using a dropper and a weighing scale.
Finally, pour 50 grams of petroleum diesel in the third measuring cylinder using a using
a dropper and a weighing scale.
Observe the volume of biodiesel fuel produced compared to the volume of the soybean oil
used to make the fuel.
Record your observations along with following measurements.
volume of the soybean oil used to make biodiesel, volume of biodiesel produced, and volume of
petroleum diesel.
Now carefully pour biodiesel into petroleum diesel until the mixture weights 100 grams
and measure the volume of the fuel mixture.
Calculate the density of the soybean oil, biodiesel, and petroleum diesel.
Thank you for watching the “Laboratory 8: Density Measurement of Chemicals and Fuels”
lab on the fundamentals of biodiesel.
Please go to the biodiesel education website for a complete written transcript of this
lab exercise.