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How Many Carbon Atoms? |
"My son has asked you to find out how many carbon atoms there are in a number of important biological molecules found in cells.
"He agrees with the new and exciting work carried out by Doktors Wohler and Liebig in 1832, which seems to suggest that oil of bitter almonds could be converted into a series of compounds containing seven carbon atoms."
"Later, in 1834, Herr Liebig discovered that alcohol, ether and muriatic ether contained as part of their structure two atoms of carbon linked to five atoms of hydrogen. All of this work suggests to my son that molecules found in living cells all have carbon atoms as part of their structure.
"He wants to know how many.
"My son is going to send you to the laboratory of the famous Herr Doktor Liebig in Munich to perform some experiments.
"You will take with you some pure samples of these biological molecules and the only thing you know about them is their molecular weight.
"In Doktor Liebig's laboratory you will conduct a series of experiments in which you will burn carefully weighed samples of your molecules in air. This will convert all the carbon atoms in your molecule into carbon dioxide.
"You will then collect all the carbon dioxide produced during the chemical reaction and weigh it. This result will become part of your data.
"You will then repeat the experiment using a different amount of your molecule and again record the amount of carbon dioxide produced. Do this several times for each sample and for each different molecule.
"You now have several sets of data and must plot some graphs.
"Along the left axis of your graph you should put a scale that shows the amount of sample burned in air.
"Along the bottom axis of your graph you should put a scale that shows the amount of carbon dioxide produced.
"Plot your data points very, very carefully and then use a ruler to draw a straight line that connects all the points. Warning! Do this part very accurately or you will not get a good result!
"From your graphs you must calculate the "slope" of each line. As in the example given below.
The "slope of a line" is calculated by dividing the vertical change in the distance between two points by the horizontal change in distance between the same two points.
"The following table gives you the atomic masses (Atomic Weights) of several important atoms.
| Element | Symbol | Atomic Mass (Weight) |
| Hydrogen | H | 1 |
| Oxygen | O | 16 |
| Carbon | C | 12 |
"A molecule of carbon dioxide has in its structure one atom of carbon and two molecules of oxygen.
"You now know five things:-
- The fact that your biological molecule has carbon atoms.
- The molecular weight of your molecule.
- The molecular weight of carbon dioxide (work it out!).
- The slope of your graph.
- What happens when your molecule is burned in air (see the formula given below).
"You must now do a calculation.
"Using the mathematical formula given below, calculate the number of carbon atoms in each of your samples. Write down your results and put them in the team portfolio".
EXTRA CREDIT! Extra credit will be awarded to any team who can answer the question .. "Why does the mathematical formula give you the number of carbon atoms in the biological molecule?"