There are 10 mandalas in the Rig Veda.

The hymn attributed to Vagambhrini in the Rig Veda is known as the Vagambhrini Sukta. It is part of the 10th Mandala (book) of the Rig Veda and is dedicated to the goddess Saraswati, who represents knowledge, language, music, and all forms of art.

A mandala could be considered as a chapter. For example mandala 1 is almost the same as saying chapter 1.

Yes, the Gayatri Mantra is considered the mother of the Vedas in Hindu tradition. It is a highly revered mantra that is found in the Rigveda and is regarded as one of the most powerful and sacred hymns.

The Burial Hymn of the Rig Veda is a funeral hymn that expresses hope for the deceased to find peace in the afterlife. It offers prayers for the departed to be guided on their journey to the next world and to be reunited with their ancestors. The hymn also reflects on the transient nature of life and the belief in the cycle of birth and rebirth.