Answers:
Trial 1: 12.49 mL = 0.0025 mol NaOH
Trial 2: 12.32 mL = 0.0025 mol NaOH
Trial 3: 11.87 mL = 0.0024 mol NaOH
Second step answers:
Trial 1 (20.61 mL HCl): 0.121 M
Trial 2 (20.06 mL HCl): 0.125 M
Trial 3 (19.67 mL HCl): 0.122 M
The average concentration (molarity) of HCl for all three trials is 0.123 M.
The percent error is 2.5%.