In Pride and Prejudice, in what chapter does Charlotte tell Elizabeth about why she is marrying Mr. Collins?
It is in Chapter XXII of Volume I (Chap. 22) that Charlotte tells Elizabeth that she has accepted Mr. Collins' offer of marriage. Charlotte never tells Mr. Collins her reasons for marrying him. In this chapter, Sir and Mrs. Lucas talk about the advantages of Charlotte's marrying Mr. Collins. It is also in this chapter that the narrator discussed Charlotte's general views of her life, prospects for happiness and ideas about marriage.
It is an oversimplification to suggest that Charlotte is marrying Mr. Collins for his money. In this era, marriage settlements still required the bride's family contribute something to the family income, which was deposited in a bank to collect interest that was used for annual household expenses: The large amount of money was left intact and the interest from it provided the income.
The Lucases were not rich and could not "settle" any significant amount of money on Charlotte at her marriage. It was therefore crucial for her to marry someone whose own money was sufficient to support a household. This wasn't considered marrying for money in the way that Wickham actively pursued marrying for money. On the contrary, this was considered prudent and wise and was advised just as Mrs. Gardiner advised Elizabeth to avoid a penniless marriage to Wickham.
Charlotte reminds Elizabeth that she has never been romantic in her ideas of love and marriage, maybe because whe was never a beauty. She tells Elizabeth that the facts that Mr. Collins has a career, connections of importance in society and a descent home are enough to fulfill her idea of the best chance for being happy in marriage. She had already told Elizabeth in Chapter VI of Volume I (Chap. 6) that she believed people could marry on little knowledge of each other and have as great a chance of happiness as people very well acquainted with each other.
To summarize, Charlotte doesn't marry Collins for his money, for one thing, he's a clergyman, he's not wealthy. She marries him because she believes happiness in marriage is a gamble even in the best situations. She also marries him because she is twenty-seven, not very pretty and living in the countryside of England with no options for love and marriage other than the ones that come to her own front door (they are not rich enough to spend time in London where she might have met more men). She marries him because it is better to have her own family and her own home than to live all her life in her parents home being always financially dependent upon them. An advantage to marrying Collins is that his connection with Lady De Bourgh means that he has the possibility for advancement and additional earnings, but this is what Charlotte's parents point out, not Charlotte.