A 15 mL sodium hydroxide solution of unknown concentration is titrated with 34.5ml of 1.2M HCl solution. What was the concentration of the sodium hydroxide solution?

NaOH + HCl = NaCl + H20

From the reaction, it can be seen that KOH and HCl have the following amount of substance relationship:

n (NaOH) : n (HCl) = 1:1

From the relationship we can determinate that NaOH and HCl have the same number of moles:

n (NaOH) = n (HCl) = c*V=1.2M*0.0345L = 0.00414 mole

Now, when moles of NaOH is known, concentration can be calculated:

c (NaOH) = n/V = 0.00414mole/0.015L = 0.276 M