Determine the approximate Ka for the indicators neutral red and chlorophenol red.

For each of these I found the midpoint in the pH range (7.4 for neutral red) and because the Ka of an indicator is equal to the concentration of H+ ions, I converted the pH to the concentration of H+ using 10^(-7.4), giving an answer of about 4x10^(-8). However, the answer key says the correct number is 8x10^(-8). Does anyone know what I'm doing wrong here? Thanks.

Question

Determine the approximate Ka for the indicators neutral red and chlorophenol red. 

For each of these I found the midpoint in the pH range (7.4 for neutral red) and because the Ka of an indicator is equal to the concentration of H+ ions, I converted the pH to the concentration of H+ using 10^(-7.4), giving an answer of about 4x10^(-8). However, the answer key says the correct number is 8x10^(-8). Does anyone know what I'm doing wrong here? Thanks.