To measure the amount of nickel in some industrial waste fluid, an analytical chemist adds 0.110 M sodium hydroxide (NaOH) solution to a 25.0 g sample of the fluid and collects the solid nickel(I) hydroxide (Ni (OH2) product. When no more Ni(OH)2 is produced, he filters, washes and weighs it, and finds that 343. mg has been produced The balanced chemical equation for the reaction is: Ni2+(aq) + 2NaOH(aq) Ni(OH)2(s) + 2 Na. What kind of reaction is this?
a. precipitation
b. acid-base
c. redox

a. precipitation

Explanation:

In the reaction:

Ni²⁺(aq) + 2NaOH(aq)→ Ni(OH)₂(s) + 2 Na⁺

Nickel is precipitated as Ni(OH)₂ producing Na⁺ ions.

In a precipitation two soluble ions react with each other to produce an insoluble salt.

In our problem, Ni²⁺ is reacting with OH⁻ to produce Ni(OH)₂, an insoluble salt

The reaction is a precipitation

In an acid-base reaction, an acid reacts with a base to produce water and a salt. In this problem is a base reacting (NaOH) but there are no acids in the reaction.

And in a redox reaction an ion is been oxidized whereas the other is been reduced (Loosing electrons and gaining electrons).

As you can see, Ni²⁺ still with oxidation number of 2+ in Ni(OH)₂ and Na⁺ is always +1-. Thus, this is not a redox reaction.