B-Mercaptoethanol is available as a 98% (w/w) solution, sp gr 1.114. What voulume is needed to prepare 0.100 L of a 50.0 mM solution. fw=78.13

I would first determine the molarity of the mercaptan.

mass of 1L = 1.114g/mL x 1000 mL x 0.98 = ??grams.
moles = ?grams/molar mass and since that is in 1L, the molarity is that same number.
Then use the dilution formula.
M1 x L1 = M2 x L2.