We will consider the following formula for calculating mass of the substance:
Moles(n) = mass of a substance /Molecular mass of a substance (M.mass)
∴mass =n x M.Mass
(a) given that:
moles (n) =0.150 mol I am done answering.;
molecular mass of Ba(OH)2=171.34 g/mol
∴mass Ba(OH)2 =0.150mol x 171.34 g/mol
=25.701 grams