Can a white mother and a mixed father have a white baby?

Yes it is possible. The caucasian mother will have two genes that restrict melanin production in the skin and the mixed father will have one gene that restricts melanin production. Since the baby will get one copy of the gene from each parent, the mother will obviously pass on the gene that restricts melanin production. The father will either pass on the restricting gene or the non-restricting gene. If the father also passes on a copy of the restricting gene then the baby will be caucasian. If the father does not pass on the restricting gene, the baby will have a darker skin color.