Final answer:
Frederick Douglass's purpose in this excerpt is to provide an example of the torment endured by enslaved persons and to highlight the injustices and cruelty of slavery, particularly with regards to his grandmother.
Step-by-step explanation:
In this excerpt from Narrative of the Life of Frederick Douglass, Douglass is expressing his deep conviction about the terrible treatment of enslaved persons and the ingratitude of slaveholders towards his grandmother.
His purpose is to provide an example of the torment that enslaved persons endure, highlighting the brutal and dehumanizing nature of slavery. He emphasizes the significance of his grandmother's faithful service and portrays the injustice and cruelty of her remaining a slave even after the death of her master.