Dados:

massa = NaOH ?

Massa molar do NaOH = 40 g/mol

Volume = 500 mL = 0,5 L

Molaridade = 2,0 mol/L

M = m / mm x v

2,0 = m / 40 x 0,5

2,0 = m / 20

m= 2,0 x 20

m= 40 g de NaOH