Why is it a good idea to wash out the tubes with the solution you are using before refilling them to read in the spectrophotometer?

because you are trying to form a chemical reaction I think

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