A 0.18 g sample of unknown was placed in a flask and dissolved in 125 mL of deionized water. A solution of HCl (0.13 M) was used to titrate the sample, and 16 mL was required to reach the endpoint. What is the formula weight of the unknown?

Answer 1

You can't do this without knowing that the sample was monobasic or not. I will assume it is.
mols HCl = M x L = ?
mols base = same.
mols = grams/molar mass. You know mols and grams, solve for molar mass.