1. (TCO 4) A chemist found a bottle of an unknown concentration of HCl solution

Question

1. (TCO 4) A chemist found a bottle of an unknown concentration of HCl solution sitting on the shelf in the lab. She found that the pH was 2.0. (a) Is the solution an acid or a base? How do you know? (10 points) (b) What is the concentration of H3O+ ions in the solution? (5 points) (c) The chemist decided to titrate the unknown solution with NaOH to determine its concentration. She placed a 25.0 mL sample of the solution into a flask and mixed in a few drops of phenolphthalein. She then slowly added NaOH and found that 18.3 mL of 2.5 M NaOH was needed to neutralize the HCl. What is the concentration of HCl in the unknown solution? (10 points) (Points : 25)

Explanation / Answer

(a) solution is acidic . pH < 7 solution is acidic pH > 7 solution basic . pH = 7 solution neutral

(b) pH = -log[H3O+]

[H3O+] = 10^-pH = 10^-2 = 1.0 x 10^-2 M

(c) mole of acid = moles of base

   M1 V1 = M2 V2

M1 x 25 = 2.5 x 18.3

M1 = 1.83 M

HCl concentration = 1.83 M

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