Answer:
0.4484 mol NaOH
Explanation:
To put it in simple terms, titration involves having the same number of moles of acid as the base. Therefore, you need to know the number of moles of the acid given the molarity and volume used, to know the molarity of base given its volume.
So nH3PO4 = 0.3810M*25ml = 9.525mol H3PO4 = 9.525mol NaOH
MNaOH = 9.525mol / 21.24ml = 0.4484mol NaOH