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pH

Does adding distilled water to a weak acid solution change its pH? Explain your answer.

Im thinking No, or if any change that does occur is negliable. weak acid dissociates as HA <==> H+ + A- where A- cud b Ch3COO-
If u add distilled water..u get HA + H2O <==> H3O+ + A- ..i think lol :confused:

is that a good enff explaination if its right?
any help wud b appreciated :biggrin:

Thanks dudes!

Reply 1

charco

Study Forum Helper

yes it changes its pH ... but not in a linear fashion (or even a log linear fashion).

It's a little confusing but there are two effects at work:
1. dilution lowers the hydrogen ion concentration raising the pH
2. dilution pushes the weak acid equilibrium further to the RHS increasing the hydrogen ion concentration thus lowering the pH.

the sum total of these two effects is dependent on the ka value of the weak acid

Reply 2

jay

what does ka value mean?

Reply 3

Aired

K_a = [H⁺][A^-]/[HA], which for weak acids is basically [H⁺]²/Molarity (The concentrations of H⁺ and A^- are the same, and as the acid is only partially dissociated, the concentration of HA will be roughly equivilent to the concentration of the acid).