There is no mention of Emily purchasing arsenic in "A Rose for Emily."
The story revolves around the mysterious and isolated life of Emily Grierson, a Southern woman from a prominent family, and the strange events that unfold in her home after the death of her father.
The confusion might stem from the fact that the story mentions Emily's purchase of poison for rats. However, this is not arsenic.
The story is deliberately vague, and Faulkner leaves the reader to speculate about Emily's motivations and the reasons behind her actions.