Which of the following salt solutions has $pH < 7$?
This question has multiple correct options
A) \[N{H_4}F\]
B) \[Cr{\left( {N{O_3}} \right)_3}\]
C) \[\left[ {{{\left( {C{H_3}} \right)}_3}N{H_4}^ + } \right]C{l^ - }\]
D) \[Ca{I_2}\]

Hint: We need to know that pH tells us whether an aqueous solution is acidic or alkaline. A solution is said to be acidic if it has pH value less than seven, whereas a solution is alkaline if it has a pH value more than seven. Neutral solutions usually have a pH of $7$. This question has multiple correct options to solve carefully.

Complete answer:
In the above question it is asked to give a salt solution having $pH < 7$, this means that we need to find a solution which is acidic in nature as it has pH value less than seven. So we will look at all the given options one by one:
Option A) this option is correct as ammonium fluoride has a pH value less than $7$ thus it is found to be acidic in nature.
Option B) This option can be correct as chromium nitrate has a pH value less than $7$, thus it is acidic in nature.
Option C) this option can be correct as \[\left[ {{{\left( {C{H_3}} \right)}_3}N{H_4}^ + } \right]C{l^ - }\], trimethyl ammonium chloride has a pH value of $5$, which is acidic in nature thus this option is correct.
Option D) this option is incorrect as calcium iodide is an inorganic salt having a pH value of approximately $7$, thus it is a neutral inorganic salt.

So, the correct answer is “Option A, B and C”.

Note:
 We need to know that the pH scale is logarithmic and inversely indicates the concentration of hydrogen ions in the solution. This is because the formula used to calculate pH approximates the negative of the base $10$ logarithm of the molar concentration of hydrogen ions in the solution. More precisely, pH is the negative of the base $10$ logarithm of the activity of the ${H^ + }$ ion.