301. A \(17.2\,\mathrm{g}\) sample of a saturated acyclic ketone gives \(12.0\,\mathrm{g}\) of ethanoic acid and \(14.8\,\mathrm{g}\) of propanoic acid on quantitative vigorous oxidation:
\[
\mathrm{Ketone+3[O]\longrightarrow CH_3COOH+CH_3CH_2COOH}
\]
Which pair gives the molecular formula of the ketone and the mass of oxygen represented by \([O]\) that is consumed?
ⓐ. \(\mathrm{C_5H_{10}O}\) and \(9.60\,\mathrm{g}\)
ⓑ. \(\mathrm{C_6H_{12}O}\) and \(12.8\,\mathrm{g}\)
ⓒ. \(\mathrm{C_5H_{10}O}\) and \(4.80\,\mathrm{g}\)
ⓓ. \(\mathrm{C_4H_8O}\) and \(6.40\,\mathrm{g}\)
Correct Answer: \(\mathrm{C_5H_{10}O}\) and \(9.60\,\mathrm{g}\)
Explanation: \( \textbf{Moles of ethanoic acid:} \)
\[
n=\frac{12.0}{60.0}=0.200\,\mathrm{mol}
\]
\( \textbf{Moles of propanoic acid:} \)
\[
n=\frac{14.8}{74.0}=0.200\,\mathrm{mol}
\]
The equation produces one mole of each acid per mole of ketone.
\[
n(\mathrm{ketone})=0.200\,\mathrm{mol}
\]
\( \textbf{Ketone molar mass:} \)
\[
M=\frac{17.2}{0.200}=86.0\,\mathrm{g\,mol^{-1}}
\]
For a saturated acyclic ketone,
\[
M=14n+16
\]
\[
14n+16=86
\]
\[
n=5
\]
Thus, the molecular formula is \(\mathrm{C_5H_{10}O}\).
The oxygen-atom amount consumed is
\[
n([O])=3(0.200)=0.600\,\mathrm{mol}
\]
\[
m(\mathrm{O})=0.600\times16.0=9.60\,\mathrm{g}
\]
The acid carbon counts also add to five, agreeing with the derived ketone formula.
302. Use the graph description below. Reaction rate is plotted on the vertical axis and oxidising-condition severity on the horizontal axis for an ordinary ketone. The curve remains close to zero under mild conditions and rises sharply only at high temperature and strong oxidant concentration. The best interpretation is:
ⓐ. Carbon-carbon cleavage appears only under vigorous oxidation
ⓑ. Stronger conditions reduce the ketone directly to an alcohol
ⓒ. The ketone is oxidised rapidly even under the mildest conditions
ⓓ. The ketone remains completely unreactive even under vigorous oxidation
Correct Answer: Carbon-carbon cleavage appears only under vigorous oxidation
Explanation: The nearly flat initial region represents the usual resistance of ketones toward mild oxidation. A substantial reaction rate appears only when the oxidising conditions become severe. Such conditions can overcome the barrier associated with cleavage of a carbon-carbon bond near the carbonyl group. The products are commonly smaller oxygenated fragments such as carboxylic acids. The graph distinguishes resistance under ordinary conditions from absolute inability to oxidise.
303. Under extremely vigorous oxidation, propanone is assumed to follow:
\[
\mathrm{CH_3COCH_3+4[O]\longrightarrow CH_3COOH+CO_2+H_2O}
\]
An \(11.6\,\mathrm{g}\) sample undergoes \(80.0\%\) conversion. Ethanoic acid is recovered in \(75.0\%\) yield. Which pair gives the recovered acid mass and the carbon dioxide volume at \(300\,\mathrm{K}\) and \(1.00\,\mathrm{atm}\)? Use \(R=0.0821\,\mathrm{L\,atm\,mol^{-1}\,K^{-1}}\).
ⓐ. \(9.60\,\mathrm{g}\) and \(3.94\,\mathrm{L}\)
ⓑ. \(7.20\,\mathrm{g}\) and \(3.94\,\mathrm{L}\)
ⓒ. \(7.20\,\mathrm{g}\) and \(2.95\,\mathrm{L}\)
ⓓ. \(9.60\,\mathrm{g}\) and \(4.92\,\mathrm{L}\)
Correct Answer: \(7.20\,\mathrm{g}\) and \(3.94\,\mathrm{L}\)
Explanation: \( \textbf{Initial propanone amount:} \)
\[
n=\frac{11.6}{58.0}=0.200\,\mathrm{mol}
\]
\( \textbf{Propanone oxidised:} \)
\[
n=0.200\times0.800=0.160\,\mathrm{mol}
\]
One mole of oxidised propanone gives one mole of ethanoic acid.
\[
n_{\mathrm{acid,formed}}=0.160\,\mathrm{mol}
\]
\( \textbf{Acid recovered:} \)
\[
n=0.160\times0.750=0.120\,\mathrm{mol}
\]
\[
m=0.120\times60.0=7.20\,\mathrm{g}
\]
One mole of oxidised propanone also gives one mole of carbon dioxide.
\[
n(\mathrm{CO_2})=0.160\,\mathrm{mol}
\]
\[
V=\frac{nRT}{P}
\]
\[
V=\frac{(0.160)(0.0821)(300)}{1.00}=3.94\,\mathrm{L}
\]
Gas formation depends on chemical conversion, not on the later acid-recovery efficiency.
304. Consider the following statements about ketone oxidation.
Statement I: Ordinary ketones generally resist mild oxidation.
Statement II: Strong oxidation may cleave carbon-carbon bonds and form smaller carboxylic acids.
Statement III: Every unsymmetrical ketone always gives one single carboxylic acid as the exclusive product.
ⓐ. Statements I and III only
ⓑ. Statements II and III only
ⓒ. Statements I, II, and III
ⓓ. Statements I and II only
Correct Answer: Statements I and II only
Explanation: Statement I describes the usual behaviour of ordinary ketones toward mild oxidants. Statement II recognises that sufficiently vigorous conditions can break the carbon skeleton. Statement III is too absolute because unsymmetrical ketones may yield more than one acid fragment, and severe oxidation can sometimes produce carbon dioxide from terminal fragments. Product prediction requires the ketone structure and the stated oxidation conditions. Ketone resistance should not be confused with complete immunity toward oxidation.
305. A mixture contains ethanal and methanoic acid. Treatment with excess \(\mathrm{NaHCO_3}\) releases \(2.46\,\mathrm{L}\) of carbon dioxide at \(300\,\mathrm{K}\) and \(1.00\,\mathrm{atm}\). A separate identical mixture gives \(64.8\,\mathrm{g}\) of silver with Tollens reagent. Assume that each mole of ethanal or methanoic acid reduces two moles of silver. Which pair gives the original mixture mass and the mole percentage of ethanal? Use \(R=0.0821\,\mathrm{L\,atm\,mol^{-1}\,K^{-1}}\).
ⓐ. \(8.80\,\mathrm{g}\) and \(50.0\%\)
ⓑ. \(18.0\,\mathrm{g}\) and \(33.3\%\)
ⓒ. \(13.4\,\mathrm{g}\) and \(33.3\%\)
ⓓ. \(13.4\,\mathrm{g}\) and \(66.7\%\)
Correct Answer: \(13.4\,\mathrm{g}\) and \(66.7\%\)
Explanation: Only methanoic acid reacts with bicarbonate to release carbon dioxide.
\[
n(\mathrm{CO_2})=\frac{PV}{RT}
\]
\[
n=\frac{(1.00)(2.46)}{(0.0821)(300)}
\]
\[
n\approx0.100\,\mathrm{mol}
\]
Therefore,
\[
n(\mathrm{HCOOH})=0.100\,\mathrm{mol}
\]
\( \textbf{Silver amount:} \)
\[
n(\mathrm{Ag})=\frac{64.8}{108}=0.600\,\mathrm{mol}
\]
Each reducing molecule produces two moles of silver.
\[
n_{\mathrm{total,reducing}}=\frac{0.600}{2}=0.300\,\mathrm{mol}
\]
\[
n(\mathrm{ethanal})=0.300-0.100=0.200\,\mathrm{mol}
\]
\( \textbf{Mixture mass:} \)
\[
m=(0.200)(44.0)+(0.100)(46.0)=13.4\,\mathrm{g}
\]
\( \textbf{Ethanal mole percentage:} \)
\[
\frac{0.200}{0.300}\times100=66.7\%
\]
The bicarbonate result measures the acid alone, whereas Tollens reagent measures both reducing components.
306. Assertion: Methanoic acid can give a positive Tollens test even though it is a carboxylic acid.
Reason: Methanoic acid can be oxidised further to carbon dioxide and therefore acts as a reducing agent.
ⓐ. Both Assertion and Reason are true, but Reason does not explain Assertion
ⓑ. Both Assertion and Reason are true, and Reason explains Assertion
ⓒ. Assertion is true, but Reason is false
ⓓ. Assertion is false, but Reason is true
Correct Answer: Both Assertion and Reason are true, and Reason explains Assertion
Explanation: Most carboxylic acids are not readily oxidised further under Tollens-test conditions. Methanoic acid is an important exception because its carbon atom can be oxidised from the carboxylic-acid level to carbon dioxide. During this process, silver(I) ions are reduced to metallic silver. The positive result therefore arises from reducing behaviour rather than from the presence of an aldehyde functional group. The Reason directly accounts for the exceptional test result.
307. Propanal vapour occupies \(4.92\,\mathrm{L}\) at \(300\,\mathrm{K}\) and \(1.00\,\mathrm{atm}\). In a Tollens test, \(80.0\%\) of the propanal reacts. Metallic silver is recovered with \(90.0\%\) efficiency, and propanoic acid after acidification is recovered in \(75.0\%\) yield. Which pair gives the recovered silver mass and recovered acid mass? Use \(R=0.0821\,\mathrm{L\,atm\,mol^{-1}\,K^{-1}}\).
ⓐ. \(34.6\,\mathrm{g}\) and \(11.1\,\mathrm{g}\)
ⓑ. \(31.1\,\mathrm{g}\) and \(11.1\,\mathrm{g}\)
ⓒ. \(31.1\,\mathrm{g}\) and \(8.88\,\mathrm{g}\)
ⓓ. \(27.6\,\mathrm{g}\) and \(8.88\,\mathrm{g}\)
Correct Answer: \(31.1\,\mathrm{g}\) and \(8.88\,\mathrm{g}\)
Explanation: \( \textbf{Initial propanal amount:} \)
\[
n=\frac{PV}{RT}
\]
\[
n=\frac{(1.00)(4.92)}{(0.0821)(300)}
\]
\[
n\approx0.200\,\mathrm{mol}
\]
\( \textbf{Propanal reacting:} \)
\[
n=0.200\times0.800=0.160\,\mathrm{mol}
\]
Each mole of aldehyde produces two moles of silver.
\[
n_{\mathrm{Ag,theoretical}}=2(0.160)=0.320\,\mathrm{mol}
\]
\( \textbf{Silver recovered:} \)
\[
n=0.320\times0.900=0.288\,\mathrm{mol}
\]
\[
m=0.288\times108=31.1\,\mathrm{g}
\]
Theoretical propanoic acid amount is \(0.160\,\mathrm{mol}\).
\[
n_{\mathrm{acid,recovered}}=0.160\times0.750=0.120\,\mathrm{mol}
\]
\[
m=0.120\times74.0=8.88\,\mathrm{g}
\]
The separate recovery factors must not be combined before applying their different stoichiometric paths.
308. The following observations are recorded with Tollens reagent.
| Row | Compound | Expected observation or product |
|---|
| P | Ethanal | Silver deposition and ethanoate formation |
| Q | Propanone | No silver mirror under ordinary conditions |
| R | Benzaldehyde | Always negative because it is aromatic |
| S | Methanoic acid | Silver deposition because further oxidation is possible |
Which combination contains all the correct rows?
ⓐ. P, Q, and S
ⓑ. P and R only
ⓒ. Q and R only
ⓓ. P, R, and S
Correct Answer: P, Q, and S
Explanation: Ethanal is oxidised to ethanoate in the alkaline reagent while silver(I) is reduced to silver metal. Propanone is an ordinary ketone and generally gives no silver mirror. Benzaldehyde is an aldehyde and normally gives a positive Tollens test despite being aromatic, so Row R is false. Methanoic acid is a reducing exception because it can be oxidised to carbon dioxide. Rows P, Q, and S reflect the correct reaction boundaries.
309. A \(250\,\mathrm{mL}\) Tollens-reagent sample contains \(0.800\,\mathrm{mol\,L^{-1}}\) reducible silver(I) complex. It is added to \(8.80\,\mathrm{g}\) of ethanal. Each mole of ethanal reduces two moles of silver(I). The metallic silver is collected with \(90.0\%\) recovery. Which pair gives the collected silver mass and the mass of unreacted ethanal?
ⓐ. \(21.6\,\mathrm{g}\) and \(0\,\mathrm{g}\)
ⓑ. \(19.44\,\mathrm{g}\) and \(2.20\,\mathrm{g}\)
ⓒ. \(17.28\,\mathrm{g}\) and \(4.40\,\mathrm{g}\)
ⓓ. \(19.44\,\mathrm{g}\) and \(4.40\,\mathrm{g}\)
Correct Answer: \(19.44\,\mathrm{g}\) and \(4.40\,\mathrm{g}\)
Explanation: \( \textbf{Silver(I) amount supplied:} \)
\[
n=MV
\]
\[
n=0.800\times0.250=0.200\,\mathrm{mol}
\]
\( \textbf{Initial ethanal amount:} \)
\[
n=\frac{8.80}{44.0}=0.200\,\mathrm{mol}
\]
Two moles of silver(I) are required per mole of ethanal.
\[
n_{\mathrm{ethanal\ oxidised}}=\frac{0.200}{2}=0.100\,\mathrm{mol}
\]
Silver(I) is limiting.
\( \textbf{Silver collected:} \)
\[
n=0.200\times0.900=0.180\,\mathrm{mol}
\]
\[
m=0.180\times108=19.44\,\mathrm{g}
\]
\( \textbf{Unreacted ethanal:} \)
\[
n=0.200-0.100=0.100\,\mathrm{mol}
\]
\[
m=0.100\times44.0=4.40\,\mathrm{g}
\]
Silver-recovery loss does not imply that the corresponding aldehyde remained unoxidised.
310. A \(2.20\,\mathrm{g}\) ethanal sample is mixed with \(25.0\,\mathrm{mL}\) of Fehling solution containing \(1.20\,\mathrm{mol\,L^{-1}}\) reducible \(\mathrm{Cu^{2+}}\). Use:
\[
\mathrm{RCHO+2Cu^{2+}+5OH^-\longrightarrow RCOO^-+Cu_2O+3H_2O}
\]
If \(\mathrm{Cu_2O}\) is recovered with \(90.0\%\) efficiency, which pair gives the precipitate mass and the mass of unreacted ethanal?
ⓐ. \(2.15\,\mathrm{g}\) and \(0.880\,\mathrm{g}\)
ⓑ. \(1.43\,\mathrm{g}\) and \(1.10\,\mathrm{g}\)
ⓒ. \(1.93\,\mathrm{g}\) and \(1.54\,\mathrm{g}\)
ⓓ. \(3.86\,\mathrm{g}\) and \(0\,\mathrm{g}\)
Correct Answer: \(1.93\,\mathrm{g}\) and \(1.54\,\mathrm{g}\)
Explanation: \( \textbf{Initial ethanal amount:} \)
\[
n=\frac{2.20}{44.0}=0.0500\,\mathrm{mol}
\]
\( \textbf{Copper(II) amount:} \)
\[
n=MV
\]
\[
n=1.20\times0.0250=0.0300\,\mathrm{mol}
\]
Two moles of \(\mathrm{Cu^{2+}}\) oxidise one mole of aldehyde.
\[
n_{\mathrm{ethanal\ oxidised}}=\frac{0.0300}{2}=0.0150\,\mathrm{mol}
\]
Copper(II) is the limiting reactant.
\[
n(\mathrm{Cu_2O})=0.0150\,\mathrm{mol}
\]
\( \textbf{Recovered precipitate mass:} \)
\[
m=0.0150\times143\times0.900
\]
\[
m=1.93\,\mathrm{g}
\]
\( \textbf{Unreacted ethanal amount:} \)
\[
n=0.0500-0.0150=0.0350\,\mathrm{mol}
\]
\[
m=0.0350\times44.0=1.54\,\mathrm{g}
\]
Precipitate-recovery loss does not change the amount of ethanal chemically oxidised.
311. Methanoic acid may reduce Fehling solution under suitable conditions because it:
ⓐ. Can be oxidised further to carbon dioxide
ⓑ. Contains a methyl ketone group
ⓒ. Is converted directly into methanal by copper(II)
ⓓ. Produces metallic copper instead of copper(I) oxide
Correct Answer: Can be oxidised further to carbon dioxide
Explanation: Most carboxylic acids are already resistant to further mild oxidation. Methanoic acid is an important reducing exception. Its carbon atom can be oxidised further to carbon dioxide. During this oxidation, copper(II) is reduced to copper(I) oxide. A positive Fehling-type response from methanoic acid therefore does not prove the presence of an aldehyde group.
312. Which row correctly describes the preparation and behaviour of Fehling solution?
| Row | Fehling A | Fehling B | Positive result |
|---|
| P | Ammoniacal silver nitrate | Dilute nitric acid | Silver mirror |
| Q | Aqueous copper(I) chloride | Concentrated hydrochloric acid | Copper metal |
| R | Aqueous copper(II) sulphate | Neutral sodium chloride solution | Blue precipitate |
| S | Aqueous copper(II) sulphate | Alkaline sodium potassium tartrate solution | Brick-red \(\mathrm{Cu_2O}\) |
ⓐ. Row P
ⓑ. Row Q
ⓒ. Row R
ⓓ. Row S
Correct Answer: Row S
Explanation: Fehling A is an aqueous copper(II) sulphate solution. Fehling B contains an alkaline solution of sodium potassium tartrate. The tartrate ligand keeps copper(II) in solution in the alkaline medium. The two solutions are commonly mixed freshly before use. A reducing aliphatic aldehyde converts the blue copper(II) complex into brick-red \(\mathrm{Cu_2O}\).
313. Propanal vapour occupies \(4.92\,\mathrm{L}\) at \(300\,\mathrm{K}\) and \(1.00\,\mathrm{atm}\). In a Fehling test, \(75.0\%\) of the propanal reacts. Copper(I) oxide forms quantitatively from the reacting aldehyde, while propanoic acid after acidification is recovered in \(80.0\%\) yield. Which pair gives the \(\mathrm{Cu_2O}\) mass and recovered acid mass? Use \(R=0.0821\,\mathrm{L\,atm\,mol^{-1}\,K^{-1}}\).
ⓐ. \(28.6\,\mathrm{g}\) and \(11.8\,\mathrm{g}\)
ⓑ. \(21.5\,\mathrm{g}\) and \(8.88\,\mathrm{g}\)
ⓒ. \(17.2\,\mathrm{g}\) and \(8.88\,\mathrm{g}\)
ⓓ. \(21.5\,\mathrm{g}\) and \(11.1\,\mathrm{g}\)
Correct Answer: \(21.5\,\mathrm{g}\) and \(8.88\,\mathrm{g}\)
Explanation: \( \textbf{Initial propanal amount:} \)
\[
n=\frac{PV}{RT}
\]
\[
n=\frac{(1.00)(4.92)}{(0.0821)(300)}
\]
\[
n\approx0.200\,\mathrm{mol}
\]
\( \textbf{Propanal reacting:} \)
\[
n=0.200\times0.750=0.150\,\mathrm{mol}
\]
One mole of aldehyde produces one mole of \(\mathrm{Cu_2O}\).
\[
m(\mathrm{Cu_2O})=0.150\times143
\]
\[
m(\mathrm{Cu_2O})=21.45\,\mathrm{g}
\]
One mole of propanal also gives one mole of propanoic acid after acidification.
\[
n_{\mathrm{acid,recovered}}=0.150\times0.800=0.120\,\mathrm{mol}
\]
\[
m_{\mathrm{acid}}=0.120\times74.0
\]
\[
m_{\mathrm{acid}}=8.88\,\mathrm{g}
\]
The acid-recovery factor does not affect the amount of copper(I) oxide formed.
314. A \(0.0500\,\mathrm{mol}\) aldehyde sample is tested separately with excess Tollens reagent and excess Fehling solution. Silver is recovered with \(80.0\%\) efficiency, while \(\mathrm{Cu_2O}\) is recovered with \(90.0\%\) efficiency. Each mole of aldehyde gives two moles of silver or one mole of \(\mathrm{Cu_2O}\). Which pair gives the recovered masses?
ⓐ. \(10.8\,\mathrm{g}\) of silver and \(7.15\,\mathrm{g}\) of \(\mathrm{Cu_2O}\)
ⓑ. \(8.64\,\mathrm{g}\) of silver and \(7.15\,\mathrm{g}\) of \(\mathrm{Cu_2O}\)
ⓒ. \(8.64\,\mathrm{g}\) of silver and \(6.44\,\mathrm{g}\) of \(\mathrm{Cu_2O}\)
ⓓ. \(5.40\,\mathrm{g}\) of silver and \(6.44\,\mathrm{g}\) of \(\mathrm{Cu_2O}\)
Correct Answer: \(8.64\,\mathrm{g}\) of silver and \(6.44\,\mathrm{g}\) of \(\mathrm{Cu_2O}\)
Explanation: \( \textbf{Theoretical silver amount:} \)
\[
n(\mathrm{Ag})=2(0.0500)=0.100\,\mathrm{mol}
\]
\( \textbf{Silver recovered:} \)
\[
m=0.100\times108\times0.800
\]
\[
m=8.64\,\mathrm{g}
\]
\( \textbf{Theoretical copper(I) oxide amount:} \)
\[
n(\mathrm{Cu_2O})=0.0500\,\mathrm{mol}
\]
\( \textbf{Copper(I) oxide recovered:} \)
\[
m=0.0500\times143\times0.900
\]
\[
m=6.435\,\mathrm{g}
\]
The recovered precipitate mass is approximately \(6.44\,\mathrm{g}\).
315. A \(15.0\,\mathrm{g}\) mixture of ethanal and benzaldehyde gives \(14.3\,\mathrm{g}\) of \(\mathrm{Cu_2O}\) with excess Fehling solution. Assume only ethanal reacts. Which pair gives the mass percentage of benzaldehyde and its mole percentage in the original mixture?
ⓐ. \(50.0\%\) and \(70.7\%\)
ⓑ. \(29.3\%\) and \(50.0\%\)
ⓒ. \(70.7\%\) and \(33.3\%\)
ⓓ. \(70.7\%\) and \(50.0\%\)
Correct Answer: \(70.7\%\) and \(50.0\%\)
Explanation: \( \textbf{Copper(I) oxide amount:} \)
\[
n(\mathrm{Cu_2O})=\frac{14.3}{143}=0.100\,\mathrm{mol}
\]
Thus,
\[
n(\mathrm{ethanal})=0.100\,\mathrm{mol}
\]
\( \textbf{Ethanal mass:} \)
\[
m=0.100\times44.0=4.40\,\mathrm{g}
\]
\( \textbf{Benzaldehyde mass:} \)
\[
m=15.0-4.40=10.60\,\mathrm{g}
\]
\[
n(\mathrm{benzaldehyde})=\frac{10.60}{106}=0.100\,\mathrm{mol}
\]
\( \textbf{Benzaldehyde mass percentage:} \)
\[
\frac{10.60}{15.0}\times100=70.7\%
\]
\( \textbf{Benzaldehyde mole percentage:} \)
\[
\frac{0.100}{0.100+0.100}\times100=50.0\%
\]
Equal mole amounts can correspond to very different mass percentages.
316. The ionic equation for Fehling oxidation is:
\[
\mathrm{RCHO+2Cu^{2+}+5OH^-\longrightarrow RCOO^-+Cu_2O+3H_2O}
\]
What masses of anhydrous \(\mathrm{CuSO_4}\) and \(\mathrm{NaOH}\) supply the stoichiometric amounts of \(\mathrm{Cu^{2+}}\) and \(\mathrm{OH^-}\) needed to oxidise \(0.125\,\mathrm{mol}\) of aldehyde? Use \(M(\mathrm{CuSO_4})=160\,\mathrm{g\,mol^{-1}}\) and \(M(\mathrm{NaOH})=40.0\,\mathrm{g\,mol^{-1}}\).
ⓐ. \(20.0\,\mathrm{g}\) and \(25.0\,\mathrm{g}\)
ⓑ. \(40.0\,\mathrm{g}\) and \(25.0\,\mathrm{g}\)
ⓒ. \(40.0\,\mathrm{g}\) and \(10.0\,\mathrm{g}\)
ⓓ. \(20.0\,\mathrm{g}\) and \(12.5\,\mathrm{g}\)
Correct Answer: \(40.0\,\mathrm{g}\) and \(25.0\,\mathrm{g}\)
Explanation: \( \textbf{Copper(II) requirement:} \)
\[
n(\mathrm{Cu^{2+}})=2(0.125)
\]
\[
n(\mathrm{Cu^{2+}})=0.250\,\mathrm{mol}
\]
One mole of \(\mathrm{CuSO_4}\) supplies one mole of \(\mathrm{Cu^{2+}}\).
\[
m(\mathrm{CuSO_4})=0.250\times160
\]
\[
m(\mathrm{CuSO_4})=40.0\,\mathrm{g}
\]
\( \textbf{Hydroxide requirement:} \)
\[
n(\mathrm{OH^-})=5(0.125)
\]
\[
n(\mathrm{OH^-})=0.625\,\mathrm{mol}
\]
One mole of \(\mathrm{NaOH}\) supplies one mole of \(\mathrm{OH^-}\).
\[
m(\mathrm{NaOH})=0.625\times40.0
\]
\[
m(\mathrm{NaOH})=25.0\,\mathrm{g}
\]
The two reagent amounts must be calculated from different coefficients in the ionic equation.
317. Assertion: Benzaldehyde is usually negative toward Fehling solution in standard textbook treatment.
Reason: Aromatic aldehydes generally do not reduce the alkaline copper(II)-tartrate complex readily under the test conditions.
ⓐ. Both Assertion and Reason are true, but Reason does not explain Assertion
ⓑ. Assertion is true, but Reason is false
ⓒ. Both Assertion and Reason are true, and Reason explains Assertion
ⓓ. Assertion is false, but Reason is true
Correct Answer: Both Assertion and Reason are true, and Reason explains Assertion
Explanation: Benzaldehyde remains an aldehyde and gives a positive Tollens test. Its response to Fehling solution differs from that of common aliphatic aldehydes. Under the usual test conditions, aromatic aldehydes generally fail to reduce the copper(II)-tartrate complex sufficiently to form the brick-red precipitate. This standard distinction is useful in qualitative analysis. The Reason directly explains the negative Fehling observation.
318. Match each reagent in Column I with its principal identifying feature in Column II.
| Column I | Column II |
|---|
| P. Fehling solution | 1. Copper(II)-tartrate in alkaline medium |
| Q. Benedict reagent | 2. Copper(II)-citrate in alkaline medium |
| R. Tollens reagent | 3. Ammoniacal silver(I) complex |
| S. \(2,4\)-DNPH | 4. Coloured carbonyl hydrazone formation |
ⓐ. P-2, Q-1, R-4, S-3
ⓑ. P-1, Q-2, R-3, S-4
ⓒ. P-3, Q-4, R-1, S-2
ⓓ. P-4, Q-3, R-2, S-1
Correct Answer: P-1, Q-2, R-3, S-4
Explanation: Fehling solution uses tartrate to stabilise copper(II) in alkaline solution. Benedict reagent uses citrate for a similar complexing role. Tollens reagent contains the diamminesilver(I) complex in ammoniacal medium. \(2,4\)-DNPH reacts with aldehydes and ketones to form coloured hydrazone derivatives. The first two reagents can both produce \(\mathrm{Cu_2O}\), but their complexing ligands differ.
319. Consider the following statements about Benedict and Fehling reagents.
Statement I: Both may form brick-red \(\mathrm{Cu_2O}\) when reduced.
Statement II: Benedict reagent uses citrate, whereas Fehling solution uses tartrate as the principal complexing ligand.
Statement III: A positive Benedict test always proves that an unknown compound is a simple aldehyde and excludes reducing carbohydrates or other reducing substances.
ⓐ. Statements I and II only
ⓑ. Statements I and III only
ⓒ. Statements II and III only
ⓓ. Statements I, II, and III
Correct Answer: Statements I and II only
Explanation: Both reagents contain copper(II) in alkaline complexed form and may yield copper(I) oxide after reduction. Their principal complexing ligands differ: citrate is used in Benedict reagent and tartrate in Fehling solution. Statement III is too absolute because Benedict reagent also responds to many reducing carbohydrates and other reducing substances. A positive copper-reduction test is therefore evidence of reducing behaviour rather than conclusive identification of one exact functional group. Additional tests are needed for a secure structural conclusion.
320. Schiff reagent is best described as:
ⓐ. Alkaline copper(II)-citrate reagent giving \(\mathrm{Cu_2O}\)
ⓑ. Ammoniacal silver(I) reagent giving metallic silver
ⓒ. Fuchsin-sulphurous acid reagent giving a magenta colour
ⓓ. Iodine-alkali reagent giving yellow \(\mathrm{CHI_3}\)
Correct Answer: Fuchsin-sulphurous acid reagent giving a magenta colour
Explanation: Schiff reagent is prepared from a fuchsin-type dye treated with sulphurous acid so that its original colour is discharged. Many aldehydes restore a pink or magenta colour when they react with this reagent. Ordinary ketones generally give no immediate colour or respond much more slowly. The test is therefore useful as supporting evidence for an aldehyde. It does not involve formation of metallic silver, copper(I) oxide, or iodoform.
