Place these hydrocarbons in order of decreasing boiling point:

paraffin :
methane:
octane:
isooctane: ( also known as 2,2,4 trimethylpentane)
octadecane:

paraffin, octadecane, octane, isooctane, methane

bp increases with increasing mol mass.

So you want them highest molmass to lowest.

paraffin, octadecane, isooctane, pentane, methan

To determine the order of decreasing boiling points for these hydrocarbons, we need to consider their molecular structure and intermolecular forces. Generally, higher molecular weight and more complex molecular structures are associated with higher boiling points.

Here is the order of decreasing boiling points for the hydrocarbons you provided:

1. Octadecane: It has the highest molecular weight among the given hydrocarbons. Octadecane is a long-chain hydrocarbon with 18 carbon atoms, and its intermolecular forces (London dispersion forces) are strong due to the increased number of electrons. This leads to a high boiling point.

2. Isooctane (2,2,4 trimethylpentane): It has a branched structure, which makes it more compact compared to linear hydrocarbons of the same molecular weight. The branching reduces the surface area, resulting in weaker intermolecular forces. However, isooctane still has higher boiling point compared to the remaining hydrocarbons.

3. Octane: It is a straight-chain hydrocarbon with 8 carbon atoms. Octane has weaker intermolecular forces compared to isooctane due to the absence of branching. Therefore, it has a lower boiling point than isooctane.

4. Paraffin (Assuming you are referring to a straight-chain alkane): Paraffins are saturated hydrocarbons with low molecular weights. Methane is the simplest paraffin, with just one carbon atom. Paraffins have weak intermolecular forces, mainly London dispersion forces. The boiling point of paraffins generally increases with increasing molecular weight, so methane would have the lowest boiling point among the given hydrocarbons.

5. Methane: It is the simplest hydrocarbon with only one carbon atom. Its molecular weight is the lowest among the hydrocarbons provided, and it has weak intermolecular forces. Therefore, methane has the lowest boiling point.

So, the order of decreasing boiling points for the given hydrocarbons is as follows:
Octadecane > Isooctane > Octane > Paraffin > Methane