Answer is: molarity of solution is 0,5 mol/dm³.
m(NaOH) = 10,0 g.
V(NaOH) = 500 ml = 0,5 dm³.
c(NaOH) = ?
n(NaOH) = m(NaOH) ÷ M(NaOH).
n(NaOH) = 10,0 g ÷ 40 g/mol.
n(NaOH) = 0,25 mol.
c(NaOH) = n(NaOH) ÷ V(NaOH).
c(NaOH) = 0,25 mol ÷ 0,5 dm³.
c(NaOH) = 0,5 mol/dm³.