pH is 3.00. The number of moles of H+ ions from HCl is equal to:
50.00 × 10-3 L × 0.100 M HCl = 5.00 × 10-3 moles.
You have added 49.00 × 10-3 L × 0.100 M NaOH = 4.90 × 10-3 moles of OH- ions.
Then it remains 5.00 × 10-3 - (4.90 × 10-3) = 1.0 × 10-4 moles H+.
The H+ concentration is 1.0 × 10-4/(0.049 L + 0.050 L) = 1.0 × 10-4/(0.099 L) = 1.00 × 10-3 M. As pH = -log[H+], pH will be 3. Read more