Question

# (a) Sketch the titration curve for an alkalinity test in which the water sample had an...

(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!

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.

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