The mass of NaOH needed to prepare the solution is 5 g
This is defined as the mole of solute per unit litre of solution. Mathematically, it can be expressed as:
Molarity = mole / Volume
We'll begin by calculating the moles of NaOH. This can be obtained as follow:
Mole = Molarity x Volume
Mole of NaOH = 0.25 × 0.5
Mole of NaOH = 0.125 mole
The mass of NaOH needed can be obtained as follow:
Mass = mole × molar mass
Mass of NaOH = 0.125 × 40
Mass of NaOH = 5 g
Thus, to prepare the solution, measure 5 g of NaOH and place it in a 500 mL volumetric flask and make it up to the mark with water.
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