Can we use the atomic mass to determine the number of neutrons?

Yes. Mass=Protons+Neutrons. Take the mass - atomic number and that gives neutrons.

the dissociation of nickel hydroxide is follows:

the concentration of hydroxide ion is

the equilibrium constant value of nickel hydroxide is = 6.1 * 10^-^1^6 and thus equilibrium equation can be written as:

thus, concentration of ni is 0.23 * 10^-^4 mol /l