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Chemistry Tutorials

Having 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:

Add elementSubtract Equivalent
CH12
H12C
OCH4
NCH2
[35]ClC2H11
FCH7
SiC2H4
PC2H7


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!!!