Salt Analysis – JEE Main ChemistryPractice Questions & PYQs

The solubility of metal chlorides is determined by the balance between lattice energy and hydration enthalpy, which dictates their dissolution behavior at various temperatures. Copper(II) chloride ($CuCl_2$) acts as a highly soluble salt at room temperature, making it the suitable choice for $x$, whereas silver chloride ($AgCl$) is effectively insoluble in water at room temperature, identifying it as the sparingly soluble $y$. Lead(II) chloride ($PbCl_2$) is characterized by its distinct property of being sparingly soluble in cold water but becoming notably soluble upon heating, which fulfills the specific criteria for $z$.

Qualitative inorganic analysis classifies metal ions into specific groups based on the solubility of their salts in particular reagents, where the combination of $\mathrm{H}_2\mathrm{S}$ and $\mathrm{NH}_4\mathrm{OH}$ establishes the alkaline medium required to precipitate Group IV cations. Among the listed ions, $\mathrm{Mn}^{2+}$ is the only species that forms a sulfide precipitate under these specific conditions. As a transition metal with an electronic configuration of $3d^5 4s^2$, manganese can lose all seven of its valence electrons to achieve its highest possible oxidation state of $+7$.

In systematic qualitative analysis, Group V cations, including $Ba^{2+}$ and $Ca^{2+}$, are collectively precipitated as carbonates by adding ammonium carbonate, $(NH_4)_2CO_3$, in the presence of ammonium hydroxide and ammonium chloride. This process relies on the solubility product constants of alkaline earth metal carbonates, which are sufficiently low to ensure precipitation when the carbonate ion concentration is elevated under these alkaline conditions. Consequently, both cations are removed from the solution as their respective white carbonate precipitates, $BaCO_3$ and $CaCO_3$.

The Griess-Ilosvay test relies on the diazotization of sulphanilic acid by acidified nitrite ions to form a diazonium salt, which subsequently undergoes a coupling reaction with $\alpha$-naphthylamine to produce a characteristic red or pink azo-dye. Because this reaction sequence accurately utilizes the specified reagents and mechanism to detect the presence of $\text{NO}_2^-$, both statements are chemically correct descriptions of the test.

The apple-green flame color is a distinct diagnostic test for the presence of $\mathrm{Ba}^{2+}$ ions, which are further confirmed by the formation of a yellow precipitate of $\mathrm{BaCrO}_4$ when the group precipitate is dissolved in acetic acid and reacted with $\mathrm{K_2CrO_4}$. The appearance of a canary-yellow precipitate upon heating the sodium carbonate extract with concentrated $\mathrm{HNO_3}$ and ammonium molybdate serves as the definitive confirmatory test for the phosphate ion, $\mathrm{PO}_4^{3-}$. Combining these unique chemical signatures identifies the specific cation and anion present in the salt as $\mathrm{Ba}^{2+}$ and $\mathrm{PO}_4^{3-}$.

Identifying specific metal sulfides hinges on their distinct behavior in qualitative analysis, such as their unique solubility in aqua regia. When $\text{CoS}$ is treated with this mixture, it dissolves to produce $\text{Co}^{2+}$ ions, which are then neutralized with $\text{NH}_4\text{OH}$ and treated with potassium nitrite and acetic acid to yield a characteristic yellow precipitate of potassium hexanitrocobaltate(III). This confirmatory transformation follows the reaction $\text{Co}^{2+} + 7\text{NO}_2^- + 2\text{H}^+ + 3\text{K}^+ \rightarrow \text{K}_3[\text{Co}(\text{NO}_2)_6] + \text{NO} \uparrow + \text{H}_2\text{O}$, which identifies compound A as $\text{CoS}$.

Primary standards must possess high purity, chemical stability, and a lack of hygroscopic properties to ensure precise concentrations when preparing standard solutions. Compounds such as $\mathrm{Na}_2\mathrm{Cr}_2\mathrm{O}_7$, $\mathrm{NaOH}$, and $\mathrm{FeSO}_4 \cdot 6 \mathrm{H}_2\mathrm{O}$ fail to meet these stringent requirements because they are either unstable, difficult to obtain in a pure state, or prone to absorbing atmospheric moisture. In contrast, substances like oxalic acid and sodium tetraborate serve as reliable primary standards, making the group containing $\mathrm{Na}_2\mathrm{Cr}_2\mathrm{O}_7$, $\mathrm{NaOH}$, and $\mathrm{FeSO}_4 \cdot 6 \mathrm{H}_2\mathrm{O}$ the unsuitable choices for quantitative titrimetric analysis.

The sodium nitroprusside test serves as a specialized chemical procedure for identifying the presence of sulphide ions within an unknown sample. When sodium nitroprusside, $\mathrm{Na}_2[ \mathrm{Fe}( \mathrm{CN})_5 \mathrm{NO}]$, interacts with sulphide ions ($\mathrm{S}^{2-}$), it undergoes a characteristic complexation reaction to generate a deep violet-coloured compound denoted as $\mathrm{Na}_4[ \mathrm{Fe}( \mathrm{CN})_5 \mathrm{NOS}]$. The observation of this distinct purple colour upon adding the reagent to the sodium carbonate extract provides reliable, visual confirmation that sulphide ions are present.

Potassium ferrocyanide, $\mathrm{K}_4[\mathrm{Fe}(\mathrm{CN})_6]$, serves as a selective reagent in qualitative analysis because it reacts with specific divalent cations to generate insoluble complex salts. Among the list provided, $\mathrm{Cu}^{2+}$, $\mathrm{Ca}^{2+}$, and $\mathrm{Zn}^{2+}$ readily react with the ferrocyanide ion to form characteristic precipitates, while $\mathrm{Fe}^{3+}$ behaves differently by producing a colored solution rather than a solid residue. The remaining ions, including $\mathrm{Ba}^{2+}$, $\mathrm{Mg}^{2+}$, and $\mathrm{NH}_4^{+}$, do not participate in this specific precipitation reaction. Consequently, there are exactly 3 cations in the group that yield a precipitate during this test.

Chemical identification tests rely on specific reactivity patterns where reagents produce observable changes-such as gas evolution, color shifts, or complex formation-only when interacting with particular functional groups. Sodium bicarbonate functions as a classic diagnostic reagent for carboxylic acids, as it reacts to release carbon dioxide gas in the form of effervescence, described by the equation $R-COOH + NaHCO_3 \rightarrow RCOONa + CO_2 \uparrow + H_2O$. Similarly, phenolic hydroxyl groups are uniquely identified by their reaction with neutral ferric chloride to produce a characteristic color, while alcoholic hydroxyl groups react with ceric ammonium nitrate to form a red complex, expressed as $(\mathrm{NH}_4)_2[\mathrm{Ce}(\mathrm{NO}_3)_6] + 2\mathrm{ROH} \rightarrow [\mathrm{Ce}(\mathrm{ROH})_2(\mathrm{NO}_3)_4] + 2\mathrm{NH}_4\mathrm{NO}_3$. Unsaturation, specifically within carbon-carbon double or triple bonds, is confirmed when the deep purple color of alkaline $\mathrm{KMnO}_4$ is discharged during oxidation.

The effervescence of $\mathrm{CO}_2$ in the presence of aqueous $\mathrm{NaHCO}_3$ occurs only when a compound is a stronger acid than carbonic acid, which has a $pKa$ of approximately 6.35. While carboxylic acids readily undergo this reaction due to their lower $pKa$ values, compounds such as phenols or alcohols lack sufficient acidity and do not react with bicarbonate ions to evolve gas. Since the compound in option A is significantly less acidic than carbonic acid, it does not release $\mathrm{CO}_2$ gas.

Ammonium salts undergo a base-catalyzed reaction with sodium hydroxide to liberate ammonia gas, which corresponds to gas $X$. Upon passing this ammonia through Nessler's reagent, an alkaline solution of potassium tetraiodomercurate(II) consisting of $\mathrm{K}_2[\mathrm{HgI}_4]$ and $\mathrm{KOH}$, a characteristic reddish-brown precipitate of the iodide of Millon's base with the formula $\mathrm{NH}_2\mathrm{Hg}_2\mathrm{OI}$ is formed, identifying the reagent $Y$.

Mohr's salt, with the chemical formula $\text{FeSO}_4 \cdot (\text{NH}_4)_2\text{SO}_4 \cdot 6\text{H}_2\text{O}$, dissociates completely in aqueous solution into ferrous (\text{Fe}^{2+}$), ammonium (\text{NH}_4^+$), and sulfate (\text{SO}_4^{2-}$) ions, confirming that Statement-I is true. Regarding Statement-II, Kohlrausch's Law of independent migration of ions dictates that the molar conductivity of an electrolyte at infinite dilution is the \sum of the molar conductivities of the constituent ions, multiplied by their respective stoichiometric coefficients as present \in one formula unit of the salt. Since the dissociation yields one mole of$\text{Fe}^{2+}$, two moles of$\text{NH}_4^+$, and two moles of$\text{SO}_4^{2-}$, the correct expression for the molar conductance at infinite dilution is$x_1 + 2x_2 + 2x_3$, rendering the provided formula$x_1 + x_2 + 2x_3$ incorrect and Statement-II false.

Lassaigne's test serves as a fundamental qualitative method for identifying nitrogen, sulphur, halogens, and phosphorus in organic compounds. The procedure relies on fusing the organic substance with metallic sodium, which effectively breaks down the covalent bonds of these elements and converts them into their respective water-soluble ionic forms for further detection. Because this critical transformation is achieved specifically through the use of sodium, the assertion that magnesium is the reagent employed for this fusion process is entirely incorrect.

The color of inorganic sulphides is primarily dictated by the electronic transitions and charge transfer characteristics inherent to their specific molecular structures. Specifically, $(\mathrm{NH}_4)_2\mathrm{S}$ often presents as a colourless to yellow compound, while the arsenic sulphides $\mathrm{As}_2\mathrm{S}_3$ and $\mathrm{As}_2\mathrm{S}_5$ are distinctly yellow. Conversely, metal sulphides such as $\mathrm{PbS}$ and $\mathrm{CuS}$ are known for their black appearance, which effectively identifies $(\mathrm{NH}_4)_2\mathrm{S}$, $\mathrm{As}_2\mathrm{S}_3$, and $\mathrm{As}_2\mathrm{S}_5$ as the specific substances that exhibit yellow coloration.

Lead sulfide, $\mathrm{PbS}$, serves as the initial reactant which dissolves in nitric acid to yield the soluble intermediate lead nitrate, $\mathrm{Pb}(\mathrm{NO}_3)_2$. Adding sulfuric acid to this solution facilitates the precipitation of lead sulfate, $\mathrm{PbSO}_4$, owing to its significant insolubility in the resulting medium. Completing the sequence, the addition of a chromate reagent produces the characteristic yellow precipitate, lead chromate, $\mathrm{PbCrO}_4$. This series of reactions illustrates how lead ions are selectively precipitated from solution by leveraging the differing solubilities of these inorganic compounds.

Potassium ferrocyanide functions as an essential qualitative reagent that identifies specific metal ions through the formation of distinct, insoluble precipitates. The reaction involving copper(II) ions produces a characteristic chocolate brown precipitate of $\mathrm{Cu}_2[\mathrm{Fe}(\mathrm{CN})_6]$, while the interaction with ferric ions yields the deep Prussian blue precipitate of $\mathrm{Fe}_4[\mathrm{Fe}(\mathrm{CN})_6]_3$. Additionally, zinc ions react to form a white precipitate identified as $\mathrm{K}_2\mathrm{Zn}_3[\mathrm{Fe}(\mathrm{CN})_6]_2$, confirming that these three specific transformations accurately describe the expected chemical behavior.

Qualitative inorganic analysis identifies metal ions by observing specific chemical transformations induced by heating and selective reagents. In the Borax bead test, a metal sulfate is heated with borax to form a characteristic metaborate, following the sequence $MSO_4 \xrightarrow[\Delta]{Na_2B_4O_7} M(BO_2)_2 \rightarrow MBO_2 \rightarrow M$. The Charcoal cavity test relies on soda fusion to convert the sample into a carbonate and subsequently a metal oxide, which is further reduced to the metal as shown by $MSO_4 \xrightarrow[\Delta]{Na_2CO_3} MCO_3 \rightarrow MO \rightarrow M$. The Cobalt nitrate test is characterized by the reaction of a metal oxide with cobalt nitrate to form a double oxide, represented by the sequence $MCO_3 \rightarrow MO \xrightarrow[+\Delta]{Co(NO_3)_2} CoO \cdot MO$. Finally, the Flame test identifies metals based on the volatile chlorides formed during heating, which correspond to the reaction path $MCO_3 \rightarrow MCl_2 \rightarrow M^{2+}$.

The addition of ammonium chloride utilizes the common ion effect to suppress the dissociation of the weak base ammonium hydroxide. By introducing a surplus of ammonium ions from the salt, the equilibrium $\mathrm{NH}_4 \mathrm{OH} \rightleftharpoons \mathrm{NH}_4^{+} + \mathrm{OH}^{-}$ is shifted to the left, which effectively decreases the concentration of hydroxide ions. This precise adjustment ensures that the $[\mathrm{OH}^{-}]$ remains low enough that only group III cations precipitate, as they possess extremely low $K_{sp}$ values in the range of $10^{-38}$.

The qualitative analysis of halide ions relies on the formation of silver halide precipitates that exhibit distinct physical properties, specifically regarding their color and solubility in aqueous ammonia. While chloride ions generate a white precipitate of $\mathrm{AgCl}$ and iodide ions produce a deep yellow precipitate of $\mathrm{AgI}$, the appearance of a pale yellow precipitate that dissolves with difficulty in $\mathrm{NH}_4\mathrm{OH}$ is specific to the formation of silver bromide, represented by the equation $\mathrm{Ag}^{+} + \mathrm{Br}^{-} \longrightarrow \mathrm{AgBr}$. This unique combination of a pale yellow appearance and restricted solubility in ammonia provides the necessary evidence to identify the acidic radical as $\mathrm{Br}^{-}$.