<u>Answer:</u> The mass of sodium sulfide needed is 195000 mg.
<u>Explanation:</u>
To calculate the moles of cadmium nitrate, we use the equation:

Molarity of cadmium nitrate = 0.0100 M
Volume of cadmium nitrate = 25.0 mL = 0.025 L (Conversion factor: 1 L = 1000 mL)
Putting values in above equation, we get:

The chemical reaction of cadmium nitrate with sodium sulfide, the equation follows:

By Stoichiometry of the reaction:
1 mole of cadmium nitrate reacts with 1 mole of sodium sulfide.
So,
of cadmium nitrate will react with =
of sodium sulfide.
- To calculate the mass of sodium sulfide, we use the equation:

Moles of sodium sulfide = 
Molar mass of sodium sulfide = 78 g/mol
Putting values in above equation, we get:

Now, converting this mass into milligrams, we use the conversion factor:
1 g = 1000 mg
So, 
Hence, the mass of sodium sulfide needed is 195000 mg.