Is the following sentence missing any apostrophes? If so, where?: The volcanos ashes spread so far that peoples windows were covered with it as far away as London.
The volcano's ashes spread so far that people's windows were covered with it as far away as London.
In both cases, these words need to have apostrophes because they are possessives. The ashes are the volcano's. The windows are the people's. 's needs to be added at the end of a noun (proper or common) to show possession (unless the noun ends in s).
If a common noun ends in s, then the correct expression of possession is simply an apostrophe, such as the cats' toys, the dogs' treats.
If a person's name ends in s, such as Allis, then the correct form of possession would be 's. Allis's books would be an example.
However, if it is an extremely significant person or literary figure (by that I mean Archimedes, Jesus, Confucius, Socrates, Sophocles, Zeus, etc.), then you would add '. Confucius' teachings, Sophocles' plays, Archimedes' discoveries, etc.
That being said, I've heard (I'm not sure, there are conflicting opinions on this) that saying the "volcano's ashes" is incorrect because volcano is not alive. Some say that you can only use 's to express possession of living things (Sally's leg, the dog's tooth, etc.).