J.R. S. answered 01/20/21
Ph.D. University Professor with 10+ years Tutoring Experience
NaCl(aq) + AgNO3(aq) ==> AgCl(s) + NaNO3(aq)
moles NaCl present = 100.0 ml x 1 L/1000 ml x 0.0500 mol/L = 0.00500 moles NaCl
moles AgNO3 present = 40.0 ml x 1 L/1000 ml x 0.100 mol/L = 0.00400 moles AgNO3
moles NaCl left over after reaction = 0.00500 - 0.00400 = 0.00100 moles NaCl left over = 0.00100 moles Cl-
Final volume = 100.0 ml + 40.0 ml = 140 ml = 0.140 L
[Cl-] = 0.00100 moles/0.140 L = 0.00714 M
pCl = -log Cl- = -log 0.00714 = 2.15