what do the numbers in front of the name mean? (for instance 2 pentene or 4 nonene)
The numbers indicate the location of the functional group. Many (most?) organics exist as chains of carbon atoms so if we want to draw a correct picture of the molecule based only on the name, it's necessary to specify where on the chain to put functional groups. For alkenes, the molecule contains a double bond, so if we start with pentane:
CH3-CH2-CH2-CH2-CH3
and want to make one of those bonds a double bond (and remove 2 hydrogens) to form pentene, we could either make the first bond in the chain a double or the second, so we need to specify whether it is 1-pentene or 2-pentene. The same is true for other functional groups, 1-propanol has the -OH group on the first carbon, 2-propanol has the -OH group on the second carbon.
This comes up in your MasteringChemistry assignment, but it is not a super-important distinction for us right now. The chemical formula of 1-propanol is C3H7OH, and the chemical formula of 2-propanol is C3H7OH, so for purposes of calculating mass or moles they can be treated the same. When we start looking more closely at properties of molecules, we will start to see differences between them, but that's a few chapters away at this point.
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