Is it permissible for a mother to donate to her youngest son a portion of her inherited wealth from her father to secure his future, and how much is she entitled to donate to him from five qirats of agricultural land?

It is not permissible for a mother to favor one of her children in gifting without a legitimate Sharia-compliant justification, as this is an act of injustice and oppression. Indeed, the Prophet, peace and blessings be upon him, refused to bear witness to a gift in which a father favored one of his children over the others. The mother's desire to secure the future of the younger son, or the fact that the father married off the elder children, is not a valid justification for favoring him.