What chapter is it in "To Kill a Mockingbird" where Jem says when he grows older he wants to go somewhere and change the law?

In Ch. 23 Jem talks with Atticus about doing away with juries.  Jem thinks it's unfair that Tom was sentenced by a racist jury.  This work to enlighten Jem on several levels. 

Jem is naive.  Atticus tells him: "So far nothing in your life has interfered with your reasoning process" (220).  Atticus means that Jem is still able to think clearly.  As he grows up, and his attitudes and ideas become more influenced by his friends and associates, they may change, just as the members of the jury had their ideas shaped by the racist culture of Maycomb.

Jem also doesn't realize that women, like Miss Maudie, cannot sit on juries.  So not only is Maycomb racist but also sexist.  He also comes to see that many prominent citizens of Maycomb, like Link Deas, would not sit on juries because it would be a conflict of interest, given that he is a businessman and might offend his customers.

Finally, Jem is also able to realize that Atticus put one of the Cunninghams on the jury, despite him being part of the lynch mob.  

Ultimately, this encounter causes Jem to declare at the end of the chapter:  "I think I'm beginning to understand why Boo Radley's stayed shut up in the house all this time . . . it's because he wants to stay inside" (227).