$500\;mL$ of a sulfuric acid solution are analysed. $100\;mL$ are taken and $50 ml\;NaOH\;0.213 M$ are added. As the obtained solution shows to be basic, it is titrated exactly with $13.21\;mL\;HCl\;0.103 M$. Calculate the molarity of the initial sulfuric acid solution!
Exercise adapted from Zumdahl - Chemical Principles - Houghton Mifflin Co - Boston 3e edition