A mediator is a person who helps two parties come to an agreement. Christ, as mediator, made it possible for a better covenant between God and humans to exist. See how you translated a similar phrase in [Hebrews 8:6](../08/06.md).
"to take away the sins of those who were under the first covenant." Possible meanings are 1) here "their sins" is a metonym for the guilt of their sins. Alternate translation: "to take away the guilt of those who were under the first covenant" or 2) here "their sins" is a metonym for the punishment for their sins. Alternate translation: "to take away the punisment for sins of those who were under the first covenant" (See: [[rc://en/ta/man/translate/figs-metonymy]])
This can be stated in active form. Alternate translation: "those whom God has chosen to be his children" (See: [[rc://en/ta/man/translate/figs-activepassive]])
Receiving what God has promised believers is spoken of as if it were inheriting property and wealth from a family member. (See: [[rc://en/ta/man/translate/figs-metaphor]])