Question

equal volumes of 0.5m HCl and 0.5m NaOH are mixed. The total volume of the resulting mixture is 2 liters. the pH of the resulting solution is
A. 1
B. 2
C. 7
D. 4

Answer 1

Equal volumes of equal concentrations of strong acid and strong base are mixed. Because the volumes and molarities are equal, the number of mole NaOH and HCl are equal.
From reaction 
HCl + NaOH ---> NaCl +H2O
we can see that if number of moles HCl and NaOH are equal, they will react completely, and no acid or base is going to be in solution anymore.

So, pH=7.