We are asked which of the following salts in each pair would the solubility depend on pH?
The salt will only depend on the pH if it has an ion that is a conjugate base of a weak acid or a conjugate acid of a weak base.
A conjugate base is simply the part of the molecule left after removing an H+.
A conjugate acid is simply the molecule after adding an H+.
Weak acids are weak electrolytes that don’t completely ionize but instead reach a state of equilibrium.
However, it’s easier to identify strong acids than familiarize yourself with all the weak acids (since the common strong acids have rules and are relatively fewer).
Strong acids are considered strong electrolytes. They completely dissociate into ions when dissolved in water.
• Strong binary acids – paired with a highly electronegative atom
Strong binary acids are: HCl, HBr, and HI
• Strong oxyacids (or oxoacids) - have 2 or more oxygens than hydrogens
Oxyacids → possess H+ ion, connected to nonmetal and oxygen
Common strong oxyacids are: HNO3, HClO3, HClO4, H2SO4
Exceptions to the rule are: Oxalic acid (H2C2O4) and Iodic acid (HIO3)
▪ both have 2 more O than H but are weak acids because carbon and iodine have low electronegativity
For which salt in each of the following groups will the solubility depend on pH?
i) NaF, NaBr
ii) Ba(ClO4)2 , Ba(ClO2)2
iii) Ca(OH)2, CaI2
iv) Fe(NO3)2, Fe(CN)2
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