After mixing, which species has the dominant affect on the pH of the solution that remains?
CsOH is a strong base.

0.5 L X 1.5 M = 0.75 moles of hydroxide.  

Similarly, there are 0.5 moles of monoprotic acid.  

After mixing there will be 0.25 moles of hydroxide in 1 liter of solution. 

In other words [OH-] = 0.25M

pH is approximately 14 + log (0.25) = 13.4