Determine how many grams of sodium phosphate would be needed to prepare 455 ml of a 2.50% (m/v) sodium phosphate solution

When the m/v percentage has been given its weight / volume percentage. This means the mass of solute in g dissolved in 100 mL of solution.

the sodium phosphate solution to be prepared should have a 2.50 % (m/v) that means there should be 2.50 g of sodium phosphate in 100 mL solution.

then if 100 mL should contain - 2.50 g

then 455 mL should have - 2.50 / 100 x 455 = 11.6025 g

the mass of sodium phosphate required is 11.6 g