Jamieson, Fausset and Brown on Jeremiah 31:7

7. The people are urged with praises and prayers to supplicate for their universal restoration. Jehovah is represented in the context (Jr 31:1, 8), as promising immediately to restore Israel. They therefore praise God for the restoration, being as certain of it as if it were actually accomplished; and at the same time pray for it, as prayer was a means to the desired end. Prayer does not move God to grant our wishes, but when God has determined to grant our wishes, He puts it into our hearts to pray for the thing desired. Compare Psa 102:13-17, as to the connection of Israel's restoration with the prayers of His people (Is 62:1-6).

for Jacob--on account of Jacob; on account of his approaching deliverance by Jehovah.

among--"for," that is, on account of, would more exactly suit the parallelism to "for Jacob."

chief of the nations--Israel: as the parallelism to "Jacob" proves (compare Ex 19:5; Psa 135:4; Am 6:1). God estimates the greatness of nations not by man's standard of material resources, but by His electing favor.