25 ml of ca(oh)2 is titrated with 17.8 ml of 0.1 m hcl. what is the concentration of ca(oh)2?

To determine the concentration of the calcium hydroxide, we need to relate the acid used by using the chemical reaction involved. The chemical reaction would be 

Ca(OH)2 + 2HCl = CaCl2 + 2H2O

We calculate as follows:

Mole HCl = 0.1 M (.0178 L) = 1.78x10^-3 mol HCl
Mole Ca(OH)2 reacted = 1.78x10^-3 mol HCl ( 1 mol Ca(OH)2 / 2 mol HCl ) = 8.4x10^-4 mol

Conc'n Ca(OH)2 =   8.4x10^-4 mol / .025 L = 0.04 M