1. Understand Percent by Mass

Percent by mass is a way of expressing the concentration of a solution. It tells you the mass of solute (sodium hydroxide in this case) present in 100 g of solution.

2. Set up the Equation

* Let 'x' be the mass of sodium hydroxide (NaOH) needed.

* The total mass of the solution will be x + 200 g (NaOH + water).

We can express the percent by mass as:

(Mass of NaOH / Mass of Solution) * 100% = 4.00%

3. Solve for x

* (x / (x + 200)) * 100% = 4.00%

* 100x = 4.00x + 800

* 96x = 800

* x = 8.33 g

Therefore, you need to dissolve 8.33 grams of sodium hydroxide in 200 g of water to make a 4.00% by mass aqueous solution.