A student dissolves 0.2879 g of potassium hydrogen phtalate (KHC8H4O4) in some distilled water and adds a few drops of phenolphthalein. The solution is then titrated with a solution of NaOH. The initial buret reading is 20.51 mL and the final buret reading is 35.84 mL.

What is the concentration of hydroxide ions in the NaOH solution?