Chapter U4.20, Problem 6E

Interpretation Introduction

Interpretation: The reason for the pH of an aqueous solution of NaCl doesn’t change upon dilution needs to be explained.

Concept Introduction:The pH of an aqueous solution is calculated as follows:

pH = -log [H₃O⁺]

Where [H₃O⁺] is concentration of hydroxonium ions in the solution.

Answer to Problem 6E

The pH of the solution of NaCl doesn’t change upon dilution because the dilution doesn’t alter (increase or decrease) the H₃O+ concentration of the solution.

Explanation of Solution

Sodium chloride, NaCl ionizes in an aqueous solution as below.

  $NaCl\left(aq\right)\to N{a}^{+}\left(aq\right)+C{l}^{-}\left(aq\right)$

Since the solution is aqueous, water, H₂O is also present in the solution in large excess. However, H₂O is a poor electrolyte and remains unionized in an aqueous solution.

The dilution of a solution simply means that more water, H₂O is added to the solution so that the molar concentration of the solute (here electrolyte, NaCl) decreases; however, the amount (in moles) of NaCl remains unchanged upon dilution.

Since the dilution of the given solution of NaCl only introduces additional H₂O in the solution which remains undissociated, hence, no H₃O+ is added or removed from the solution upon dilution and thus, the pH of the solution remains unaltered.

Conclusion

The pH of the solution doesn’t change upon dilution because there is no change in the concentration of hydrogen ion takes place.