18 test tubes were put in two rows in a test tube rack. 10 mL of 50 mM HCl were put into each test tube in the front row, 10 mL of 0.1M NaOH were put into each test in the back row. Tubes in the front row were marked 1, 2, 3... up to 9. To the tube marked 1 one drop of the methyl red solution was added, two drops were added to the test tube marked 2, three drops to the test tube marked three and so on, up to nine drops in the front row test tube marked nine. Then, the same solution of the methyl red was added to NaOH filled test tubes from the back row, but in reversed order - one drop to the test tube behind the one marked 9, two drops to the test tube behind the one marked 8 and so on, up to nine drops put in the test tube behind the one marked 1. Source of a white light was put behind the rack so that it was possible to observe hue of the light after passing through both solutions.
Two additional test tubes were filled one with a pure water and the other one with unknown solution. To the one with unknown solution 10 drops of the same methyl red solution were added. When these two tubes were observed in identical conditions, observed hue was almost identical to the one observed when the light was passing through the test tubes marked with digit 3.
What is pH of the unknown solution, if pKa of the methyl red is 5.0?