Question

(a) Sketch the titration curve for an alkalinity test in which the water sample had an initial pH of 6.35. In that test, 15 mL of 0.1 N acid was added to a 150-mL sample.

(b) Calculate the alkalinity in meq/L and mg/L as CaCO₃. Answer: 10 meq/L, 500 mg/L as CaCO₃

_{I know the answer is correct but I am having a problem on a few of the steps, please show all work for a "thumbs up"! Thanks!}

Answer 1

b)

Moles of acid added = Moles of H+ added = 15 * 0.1 = 1.5 mmoles

Alkalinity in meq/L = 1.5 /150 * 1000 = 10 meq/L

Alkalinity as CaCO3 = (10*MW /n) mg/L = 10 * 100/2 = 500 mg/L

a)

pH = 6.35

6.35 = -log([H+])

Initial [H+] = 4.47 x 10^-7 M

Initial moles of H+ = 150 * 4.47 x 10^-7 = 6.7 x 10^-5 mmoles

Moles of H+ added by (V mL) of acid = 0.1 V mmoles

Total volume = (150 + V) mL

Sketching typical titration curve we get:

15 mL on Y-Axis indicates the end point.