Question

In the precipitation of the iron group in qualitative analysis, ammonium chloride is added before adding ammonium hydroxide to:
A.Decrease concentration of $O{H^ - }$ ions
B.Prevent interference by phosphate ions
C.Increase concentration of $C{l^ - }$ ions
D.Increase concentration of $NH_4^ +$ ions

Answer 1

Qualitative analysis in chemistry can be understood as the analysis of a particular species in a given chemical compound. What this actually means is that qualitative analysis is a method that is used to identify and focus on a certain part of a given chemical compound.

Complete step by step answer:
Before we move forward with the solution of the given question, let us first understand some important basic concepts.
Let us consider this case given to us. We are supposed to perform qualitative analysis for iron groups in some compounds. This means that we would be working towards the identification and analysis of the presence of iron group elements in the given compound.
Solubility products can be understood as a chemical property of a substance that describes the degree of dissolution of a compound in a solvent to form a solution. Low solubility product is an indicator of formation of undissolved solute in the solution, also known as a precipitate.
Another parameter we need to understand is common in effect. Common ion effect is defined as the analysis of a situation when a common ion is added to a given solution. In a given solution another ion similar to the solute is added, then the solubility of the solute decreases.
In the precipitation of the iron group in qualitative analysis, ammonium chloride is added before adding ammonium hydroxide to decrease the concentration of $O{H^ - }$ because of the common ion effect, i.e. $NH_4^ +$ ions.

Hence, Option A is the correct option

Note: The decrease in the $O{H^ - }$ ions help in preventing the reaction of the group 3 elements, or the iron group elements when they are undergoing qualitative analysis.