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?

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.

To find the formula weight of the unknown, we can use the concept of stoichiometry and the volume and molarity of the HCl solution used in the titration.

First, let's determine the number of moles of HCl used in the titration. We can use the equation:

moles of solute = molarity x volume (in liters)

Given:
Molarity (HCl) = 0.13 M
Volume (HCl) = 16 mL = 0.016 L

moles of HCl = 0.13 M x 0.016 L = 0.00208 moles

Since the reaction between the unknown substance and HCl is 1:1, the number of moles of the unknown substance is also 0.00208 moles.

Next, let's find the molar mass (formula weight) of the unknown substance using the given mass and the number of moles.

mass of unknown = 0.18 g
moles of unknown = 0.00208 moles

molar mass (formula weight) = mass of unknown / moles of unknown

molar mass = 0.18 g / 0.00208 moles ≈ 86.54 g/mol

Therefore, the formula weight of the unknown substance is approximately 86.54 g/mol.