Question

Each statement refers to its own solution; which of the following won’t affect the pH value of that given solution?

Adding 100 mL of deionized H₂O to a solution with pH of 2.5

Heating up the solution from room temperature to 50 oC

Preparing the solution with tap water instead of distilled or deionized water

Adding 1 gram of NaOH solid to a 100 mL of 1 mol/L NaOH solution

Mixing 25 mL of a 0.5 mol/L HCl solution with 500 mL of a 0.5 mol/L HCl solution

Answer 1

Of the given statements above, the following statements won’t affect the pH value of the given solution,

Adding 100 mL of deionized H₂O to a solution with pH of 2.5 - would dilute the solution, therefore net [H+] molar concentration would be reduced in solution due to increase in volume. pH would increase.

Heating up the solution from room temperature to 50 ^oC - would increase the rate of dissociation of acid in solution, that is [H+] concentration would rise, pH would decrease.

Preparing the solution from tap water instead of distilled water - the ions present in tap water would change the ionic strength and therefore acid dissociation in solution. pH would change.

Adding 1 gram of NaOH solid to a 100 mL of 1 mol/L NaOH solution - NaOH is a base, it would neutralize some H+ in solution and therefore, pH would increase.

Correct answer

Mixing 25 mL of a 0.5 mol/L HCl solution with 500 mL of a 0.5 mol/L HCl solution - final [H+] concentration in solution

= (0.5 M x 25 ml + 0.5 M x 500 ml)/525 ml = 0.5 M

So the initial and final concentration of [H+] in solution remained 0.5 M

Therefore, no change in pH would be seen by mixing ther two solutions.