Respuesta :

NaOH+HCl ----->NaCl+H2O

number mole NaOH = number mole NaOH
M -molarity, V - volume
M(NaOH)*V(NaOH)  =  M(HCl)*V(HCl) 
M(NaOH)*40 ml  =  0.2 M*50 ml
M(NaOH)=0.2 M*50 ml/40 ml=1/4 M=0.25 M NaOH