The amount of solid NaOH that must be added to 750g of an aqueous solution to make 2.5 percent NaOH by mass can be calculated using the following formula:

Mass of NaOH required = (Percentage of NaOH / 100) * Mass of solution

Given:

Mass of solution = 750g

Percentage of NaOH = 2.5%

Substituting the values into the formula:

Mass of NaOH required = (2.5 / 100) * 750g

Mass of NaOH required = 0.025 * 750g

Mass of NaOH required = 18.75g

Therefore, 18.75g of solid NaOH must be added to 750g of the aqueous solution to make a 2.5 percent NaOH solution by mass.