In the book, The Great Gatsby, on what page in Chapter 2 does Nick talk about the valley of ashes? ...

In the Scribner, New York, 2004, edition of the novel, the passage to which you seem to refer begins on page 23.  It is in the very first paragraph of Chapter II.  Nick says,

This is a valley of ashes -- a fantastic farm where ashes grow like what into ridges and hills and grotesque gardens; where ashes take the forms of houses and chimneys and rising smoke and, finally, with a transcendent effort, of men who move dimly and already crumbling through the powdery air.

The valley of ashes is an area between West Egg and New York City, and it really represents the terrible price of industrialization and the futility of an attempt to reach the American Dream, revealed in the novel as a fiction.  George Wilson, husband of the unfaithful and cruel Myrtle, lives here.  He tries to work hard, but it seems that he can just never get ahead, and he is taken advantage of by his hateful wife (who longs for status and money that he cannot provide) as well as Tom Buchanan (who enjoys lording his status over Wilson, figuratively dangling his car in front of the mechanic like a carrot).  Wilson's plight shows how the idea that one can achieve success and prosperity with simple hard work and determination is a fantasy.