What is the hydroxide-ion concentration in a solution formed by combining 200. ml of 0.16 m hcl with 300. ml of 0.091 m naoh at 25oc? hcl(aq) + naoh(aq) → nacl(aq) + h2o(l)?

The acid base reaction equation is as follows;
HCl + NaOH ---> NaCl + H₂O
stoichiometry of HCl to NaOH is 1:1
NaOH is a strong base and HCl is a strong acid they completely dissociate into its ions
 number of HCl moles - 0.16 mol/L x 0.200 L = 0.032 mol
number of NaOH moles - 0.091 mol/L x 0.300 L = 0.0273 mol 
Limiting reactant is NaOH therefore number of HCl moles reacted - 0.0273 mol
the excess amount of HCl moles are - 0.032 - 0.0273 = 0.0047 mol
since HCl is a strong acid 
total volume - 200 mL + 300 mL = 500 mL 
[HCl ] = [H⁺]
therefore [H⁺] = 0.0047 mol / 0.500 mL 
[H⁺] = 0.0094 mol/L
hydroxide ion concentration - 0.0094 M

It was ernest rutherford