Final answer:
The solubility guideline that 'most ionic chlorides are soluble' has exceptions for silver, lead(II), and mercury(II) salts. If one of the guidelines overlooks this, then stating that 'A-D are all valid' would be incorrect. Additionally, solubility can be affected by the presence of strong acids.
Step-by-step explanation:
The student asked which of the following water solubility guidelines is not valid: Most ionic chlorides are soluble. The solubility rule regarding ionic chlorides is that most ionic chlorides are soluble except for those containing silver, lead(II), and mercury(II). Silver chloride, for instance, dissociates to a small extent in water, forming silver ions and chloride ions, and is considered a strong electrolyte. However, when it comes to exceptions like silver chloride, lead(II) chloride, and mercury(II) chloride, the statement should read 'All are soluble except silver, lead(II), and mercury(II) salts.' Hence, given the provided solubility guidelines, the statement E 'A-D are all valid' would not hold true if any of the mentioned options (A-D) omits this exception.
Moreover, when compounds such as AgCl are dissolved in the presence of a strong acid like HClO4, the solubility can be greater than in pure water due to the common ion effect.