Just a little indication:
to get to the answer, I had to do
2 * Cca(oh)2 * Vca(oh)2 = Chcl * Vhcl
because if Ca(OH)2 = (for example) 1 mol/L, then [Ca2+]=1mol/L and [OH-]=2mol/L, right?
Therefore, it should be [Ca(OH)2)] * 2 because we want the concentratio n of the OH- ion, not the whole molecule...still right?
So concerning what I said always worked perfectly was :
5Fe2+ + MnO4- -> [...]
in this case it was Cfe * Vfe = Cmno4 * Vmno4 * 5
because if [Fe2+]=1 mol/L than 5*1mol/L=5mol/L=[MnO4]
Do I have it right? If I do, can I conclude that (in c1v1=c2v2) when the number (coefficient) is in front of the molecule, we multiply the opposite molecule by this coefficient, whereas when the number is in the molecule (as in Ca(OH)2), we multiply the concentration of the molecule by the number.
Thanks again all, I appreciate your help.