Brother Gregory wants you to help him find out how many carbon atoms there are in various samples and specimens. In these investigations you will be provided with samples of biological substances and molecules taken from unknown materials. You will be asked to find out how many carbon atoms there are in the molecular structure of these molecules.

First, print out your personal investigation page (below) and find out which specimens you are supposed to investigate. Carry out the experiment, gather and record your data, analyze your results and write down the appropriate numbers on your personal investigation page.

This is important. Bring your completed investigation page to the examination.

Brother Gregory has provided you with a number of biological molecules. He wants you to use methods devised by Herr Doktor Liebig to determine how many carbon atoms there are in each one of these molecules.

For example, there are two carbon atoms in the molecular structure of this amino acid, but how do we know? These molecules are too small to be seen with a microscope, so other methods must be used instead.

The method you are going to use was devised by Herr Doktor von Liebig.

In this procedure a sample of the unknown substance is carefully burnt in oxygen. This destroys the original molecule and produces two new molecules; water and carbon dioxide, thus ...

The carbon dioxide produced as the unknown substance burns is collected and weighed. The molecular weight of the carbon dioxide is known (or can be worked out from its formula), the molecular weight of the unknown substance is also known, so a simple calculation can be used to relate the amount of CO₂ produced to the amount of carbon in the original molecule.

As the first step in this investigation, it is necessary to burn at least two samples of each unknown substance in oxygen, then collect and weigh the amount of CO₂ given off in each case.

Procedure

1. Start the Analysis Program by clicking here.

2. Click on the button next to your substance/specimen number

3. Write down the name of the substance/specimen and write down the molecular weight. You will need this information later.

4. For each substance/specimen chose a small number of grams of sample (for example - try 10 grams). Enter this number into the appropriate box, then click on the "Analyze" button.

5. Write down the number of grams of CO₂ collected.

6. Now enter a larger number of grams in the sample box (for example - try 56 grams, or more), click on the Analyze button again, and once more write down the number of grams of CO₂ collected.

7. Save all of these numbers for the next step.

8. Repeat this procedure for every substance/specimen in your investigation assignment.

As the second step in this investigation you now have to find the relationship between the amount of sample burnt, and the amount of CO₂ produced.

Procedure

1. Start the Slope Calculator program by clicking here.

2. Using the data you collected in Part One, enter all the appropriate values into the appropriate boxes. Be careful it is easy to make a mistake and mix up the numbers. Check your work.

3. Click on the Get Run button in the middle of the page (below the entry boxes). The value of the "run" of the slope calculation will appear. This number is the difference between the smaller amount of sample and the larger amount of sample.

4. Click on the Get Height button in the middle of the page. The value of the "height" of the slope calculation will appear. This number is the difference between the lesser amount of carbon dioxide collected and the larger amount of carbon dioxide collected.

5. Now click on the Slope of Line button, and the value for the slope of this graphed line will appear. Write it down. You will need this value later.

6. Repeat these calculations for all the substances/specimens you have analyzed.

You now have two important pieces of information for each of your substances or specimens: (a) its molecular weight and (b) its relationship to the amount of CO₂ produced when a sample is burnt in oxygen.

You must now calculate the molecular weight of carbon dioxide, and use a formula to determine the number of carbon atoms in the unknown substance or specimen.

Element	Symbol	Atomic Mass
Hydrogen	H	1
Carbon	C	12
Nitrogen	N	14
Oxygen	O	16

1. Begin by calculating the molecular weight of carbon dioxide from the atomic masses of each of the atoms given in the table above.

2. Start the Carbon Atom Calculator by clicking here.

3. Use the data you collected and calculated to fill in the appropriate boxes in the calculator.

4. Click on the --=== find carbon atoms ===-- button at the bottom of the calculator and the value for the number of carbon atoms will appear in the appropriate box. Write this down (warning - it may not be a whole number!).

Find the answers to the questions, write your answers on the investigation page, and bring the completed assignment to the exam for grading purposes.

Personal Investigation Page