For solving such problems we need to know mole concept,
Here, Volume and Concentration is given so it's so easy to find number of moles. first we have to convert volume from ml to litre and and then multiply by the concentration of NaOH:
★ Volume of NaOH = 250ml = 250 × 10⁻³ = 0.25L
★ Concentration of NaOH = 0.15 mol/L
Now, let's find the number of moles:-
Number of moles = volume × concentration
Number of moles = 0.25*0.15
Number of moles = 0.0375 mol/L
Thereafter, we have to find molar weight of NaOH and for this we have to know atomic mass of these elements:-
Na = 22g
O = 16g
H = 2g
Substitute the value
Molar weight of NaOH = 22+16+2
= 40 g/mol
Mass of NaOH = no. of moles × molar weight
= 0.0375*40
= 1.5 g/mol
Hence, We need 1.5 g /mol to make NaOH.