This is a neutralization reaction with 1 mole of HCl needed to neutralize 1 mole of NaOH.

First, we determine the quantity of HCl in the solution.

The quantity of HCl used equals 17.5 * 0.001 L * 0.5 mol/L = 0.00875 moles

Hence, the amount of NaOH neutralized also equals 0.00875 moles.

The molar mass of NaOH is 40 g/mol

The mass of NaOH in the solution is: 40 g/mol * 0.00875 mol = 0.35 g.

% concentration can be found by finding the weight of NaOH per 100 g (or per 100 mL) of the solution:

0.35 g / 35 L * 1000 = 10 g/L or 1 g per 100 mL (or per 100 g solution) or 1%

Answer: 1%