Answer is: mass of sodium hydroxide is 60 grams.
V(NaOH) = 300 mL · 1 L/1000mL = 0,3 L.
c(NaOH) = 5 M = 5 mol/L.
n(NaOH) = c(NaOH) · V(NaOH).
n(NaOH) = 5 mol/L · 0,3 L.
n(NaOH) = 1,5 mol.
m(NaOH) = n(NaOH) · M(NaOH).
m(NaOH) = 1,5 mol · 40 g/mol.
m(NaOH) = 60 g.
