Concentration of the NaOH solution. So hard. Really need help!?

Question

A student is titration 25.0 cm3 of a solution of NaOH with a 0.247 M solution of HCl. Accidentally, the student adds too much HCl. The amount of HCl solution added is 34.38 cm3. Using a buret filled with the unknown NaOH solution, the student finds that the addition of 4.77 cm3 of base solution results in an end-point. What is the concentration of the NaOH solution?

Answer 1

find the moles of HCl in 34.38 ml

0.03438 Litres @ 0.247mol/Litre = 0.008492 moles

-------------------

0.008492 moles HCl = 0.008492 moles NaOH

----------------------

25.00 ml + 4.77ml = 29.77 ml NaOH

--------------------

Molarity NaOH:

0.008492 moles / 0.02977 Litres = 0.2852 molar NaOH

your answer is(3 sig figs): 0.285 Molar