Sodium salt of an organic acid 'X' produces effervescence with conc. H2SO4. 'X' reacts with the acid

Q: Why doesn't sodium acetate give effervescence with conc. H2SO4?

Sodium acetate reacts with conc. H2SO4 to form acetic acid, which gives off vinegar-smelling vapors, but it does not decompose to release gases like CO or CO2 that cause brisk effervescence.

Q: Why is the KMnO4 solution acidified in the final test?

Acidic medium provides the necessary H+ ions for the reduction of the permanganate ion (MnO4-) to the colourless Mn2+ ion. Without acid, KMnO4 might reduce to MnO2, which is a brown precipitate, masking the decolourisation.

Q: What is the oxidation state of carbon in the oxalate ion?

In the oxalate ion ($C_2O_4^{2-}$), the oxidation state of carbon is +3. During the reaction with KMnO4, it gets oxidized to CO2, where its oxidation state becomes +4.

Q: Can we use dilute H2SO4 instead of concentrated H2SO4 in the first step?

No, dilute H2SO4 is not a strong enough dehydrating agent to decompose the oxalate ion into CO and CO2 gases. Concentrated H2SO4 is specifically required for this decomposition.

Q: Why does calcium oxalate form a precipitate while sodium oxalate is soluble?

Calcium oxalate has a very low solubility product (Ksp) compared to sodium oxalate. When the ionic product of Ca2+ and C2O4 2- exceeds its Ksp, it precipitates out of the solution.

Question

Sodium salt of an organic acid 'X' produces effervescence with conc. $H_2SO_4$. 'X' reacts with the acidified aqueous $CaCl_2$ solution to give a white precipitate which decolourises acidic solution of $KMnO_4$. 'X' is

Options:

(A) $C_6H_5COONa$

(B) $HCOONa$

(C) $CH_3COONa$

(D) $Na_2C_2O_4$

Sigma Percentile · Accepted Answer

Imagine you are a chemical detective in a laboratory, and you are handed a mysterious white powder, labeled simply as 'X'. Your mission is to uncover its true identity using a series of classic chemical tests. This problem takes us through exactly such a scenario, testing our knowledge of qualitative analysis and redox reactions.

### The Mystery of the Effervescence

Our first clue comes from the reaction of salt 'X' with concentrated sulfuric acid ($H_2SO_4$). The problem states that this reaction produces brisk effervescence. Effervescence is the tell-tale sign of a gas being evolved.

When we look at the given options, we have sodium benzoate, sodium formate, sodium acetate, and sodium oxalate. When sodium oxalate ($Na_2C_2O_4$) is treated with concentrated sulfuric acid, the acid acts as a powerful dehydrating agent. It breaks down the oxalate ion, releasing a mixture of carbon monoxide ($CO$) and carbon dioxide ($CO_2$) gases.

$$Na_2C_2O_4 + H_2SO_4 \rightarrow Na_2SO_4 + H_2O + CO \uparrow + CO_2 \uparrow$$

This rapid evolution of gases is what causes the observed effervescence.

### The White Precipitate Clue

Next, the problem tells us that 'X' reacts with an acidified aqueous solution of calcium chloride ($CaCl_2$) to form a white precipitate.

When sodium oxalate is mixed with calcium chloride, a double displacement reaction occurs. The calcium ions ($Ca^{2+}$) combine with the oxalate ions ($C_2O_4^{2-}$) to form calcium oxalate ($CaC_2O_4$). Calcium oxalate is highly insoluble in water, which is why it immediately crashes out of the solution as a distinct white precipitate.

$$Na_2C_2O_4 + CaCl_2 \rightarrow CaC_2O_4 \downarrow + 2NaCl$$

This step strongly points towards the presence of the oxalate radical.

### The Ultimate Redox Confirmation

The final and most definitive test involves treating this white precipitate with an acidic solution of potassium permanganate ($KMnO_4$). Potassium permanganate is a famous oxidizing agent, known for its deep, vibrant purple color.

When the white precipitate of calcium oxalate is added to it, the purple color vanishes—it is decolourised! This happens because a redox reaction takes place. The oxalate ion acts as a reducing agent. It reduces the purple permanganate ion ($MnO_4^-$) to the colourless manganese(II) ion ($Mn^{2+}$). In the process, the oxalate ion itself gets oxidized to carbon dioxide gas.

$$5CaC_2O_4 + 2KMnO_4 + 8H_2SO_4 \rightarrow K_2SO_4 + 5CaSO_4 + 2MnSO_4 + 10CO_2 \uparrow + 8H_2O$$

**Notice the oxidation states:** Manganese goes from +7 in $KMnO_4$ to +2 in $MnSO_4$, gaining 5 electrons (reduction). Carbon goes from +3 in $C_2O_4^{2-}$ to +4 in $CO_2$, losing 1 electron per carbon atom (oxidation).

### Conclusion

By piecing together these three clues—the effervescence with concentrated sulfuric acid, the formation of a white precipitate with calcium chloride, and the decolourisation of potassium permanganate—we can confidently conclude that the unknown salt 'X' is indeed **sodium oxalate ($Na_2C_2O_4$)**.

Animated Solution for Chemistry - Redox Reactions: Sodium salt of an organic acid 'X' produces effervescence with conc. $H_{2} S O_{4}$ . 'X' reacts with the acidified aqueous $C a C l_{2}$ solution to give a white precipitate which decolourises acidic solution of $K M n O_{4}$ . 'X' is

Select Answer:

Visualized Solution

$Q u a l i t a t i v e A na l y s i s S e t u p$

$T es t 1 : E f f er v esce n ce$

$T es t 2 : W hi t e P r ec i p i t a t e$

$T es t 3 : D eco l o u r i s a t i o n o f K M n O_{4}$

$F ina l C o n c l u s i o n$

$T h e W a y F or w a r d$

The Sigma Insight: Oxidation and Reduction

The Mystery of the Effervescence

The White Precipitate Clue

The Ultimate Redox Confirmation

Conclusion

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