7. A structural isomer of pentane needs to have the same number of carbon (5) and hydrogen atoms as pentane. 2,2-dimethylbutane and 2-methylpentane have 5 carbon atoms, while octane has 8. 2-methylbutane has 5 carbon atoms and this is a structural isomer of pentane.
12. The formula that goes with the picture is CH₃(CH₂)₂COOH.
18. The difference between unsaturated and saturated fat is if there are one or more double bonds. The unsaturated fat is transformed into saturated by addition of hydrogen to the double bond, so the saturated fat are saturated with hydrogen.
21. Some of by-products of burning coal are oxides of nitrogen and sulfur, as well as different metals, such as lead, mercury, cadmium, arsenic, and many others. Coal contains more aromatic compounds that other aliphatic fuels obtained from petroleum, so it leaves more soot upon burning.
22. This is butanoic acid, and it has carboxyl (-COOH) functional group.
23. Alcohols and carboxylic acids create hydrogen bonds, so it is necessary to provide more energy to breaks this bonds too, which make their boiling points higher that they should be. On the other hand, hydrocarbons and halocarbons do not create hydrogen bonds.