
Chemistry TutorialsHaving some trouble with chemistry? Hopefully these tutorials here made by me will be of some use. Hehe, it also gives me a chance to study, since teaching others is actually the best way to learn something.Rule of 13 This rule is used when figuring out how many Carbons and Hydrogens are in a molecule. It is also used to find all the possible molecules with a certain mass. It is used in mass spectrometry.1. Take the mass of the molecule and divide it by 13 (Mass of a CH group). 2. The whole number is the number of Carbons, and the whole number plus the numerator of the remainder is the number of Hydrogens. For example, 68/13 = 5 + 3/13 5 is the number of Carbons, and 5+3=8 is the number of hydrogens. So for hydrocarbon formula for mass 68 is C5H13 3. Of course, many times there are more than just carbons and hydrogens in a molecule. Use the following chart to find possiblities with other atoms:
What this means is that if you know that there's an Oxygen in the molecule, to have a mass of 68, the formula would be C4H9O, since you would have to remove 1 carbon and 4 Hydrogens from the first formula. If you ever encounter the problem of not having enough carbons/hydrogens to subtract, you can either subtract/add hydrogens/carbons to obtain extra ones. If that doesn't work then that means that would not be an possibility. More coming soon!!! 