71.

In the equation

CrO${}_4^{2-}$ + SO${}_3^{2-} \to \mathrm{Cr}(\mathrm{OH}{)}_4^{-} + {\mathrm{SO}}_4^{2-}$

the oxidation number of Cr changes from

• +6 to +4

• +6 to +3

• +8 to +4

• +4 to +3

Which quantities are conserved in all oxidation- reduction reactions?

• Charge only

• Mass only

• Both charge and mass

• Neither charge nor mass

The oxidation number of chromium in CrO₅ is

• +6

• +5

• +10

• 0

Silver metal is recovered industrially by using the chemistry shown below. Which of the following statements is/are not true?

2K[Ag(CN)₂] +Zn→ K₂[Zn(CN)₄] + 2Ag

• only Ag has been oxidised and Zn reduced

• (a), (c), (d)

• Both Ag and Zn have been oxidised

• Both Ag and Zn have been reduced

Which of the following does not/do not represent the correct equivalent mass of Cr₂O${}_7^{2-}$ in the reaction Cr₂O${}_7^{2-}$+ 14 H⁺ + 6e^- → 2Cr³⁺ + 7H₂O.

• The molar mass of dichromate

• One third of the molar mass of dichromate

• One half of the molar mass of dichromate

• all of the above

The reaction, 10FeSO₄ + 2KMnO₄ + 8H₂SO₄ → 2MnSO₄ + 5Fe(SO₄)₃ + K₂SO₄ + 8H₂O is an example of reaction of 

• disproportionation

• intermolecular redox

• intramolecular redox

• None of these

Standard electrode potential data are useful for understanding the suitability of an oxidant in a redox titration. Some half cell reactions and their standard potentials are given below

$$\begin{gathered} {\mathrm{MnO}}_4^{-}\left(\mathrm{aq}\right) + 8{\mathrm{H}}^{+}\left(\mathrm{aq}\right) + 5{\mathrm{e}}^{-} \to {\mathrm{Mn}}^{2+}\left(\mathrm{aq}\right) + 4{\mathrm{H}}_2\mathrm{O} \left(\mathrm{l}\right): {\mathrm{E}}^{°}= 1.51\mathrm{V} \\ {\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}\left(\mathrm{aq}\right) + 14{\mathrm{H}}^{+}+ 6{\mathrm{e}}^{\mathrm{\_}} \to 2{\mathrm{Cr}}^{3+} + 7{\mathrm{H}}_2\mathrm{O} \left(\mathrm{l}\right): {\mathrm{E}}^{°}= 1.38\mathrm{V} \\ {\mathrm{Fe}}^{3+}\left(\mathrm{aq}\right) + {\mathrm{e}}^{-} \to {\mathrm{Fe}}^{2+}\left(\mathrm{aq}\right); {\mathrm{E}}^{°}=0.77\mathrm{V} \\ {\mathrm{Cl}}_2\left(\mathrm{g}\right) + 2{\mathrm{e}}^{-} \to 2{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right); {\mathrm{E}}^{°}=1.40\mathrm{V} \end{gathered}$$

Identify the incorrect statement regarding the quantitative estimation of gaseous Fe(NO₃)₂.

• MnO${}_4^{-}$ can be used in aqueous HCl

• Cr₂O${}_7^{2-}$ can be used in aqueous HCl

• MnO${}_4^{-}$ can be used in aqueous H₂SO₄

• Cr₂O${}_7^{2-}$ can be used in aqueous H₂SO₄

Oxidation number of chlorine atom in Ca(OCl)Cl is

• -1

• +1, -1

• 0

• -1, 0

The ${\mathrm{E}}_{{\mathrm{M}}^{2+}/ {\mathrm{M}}^{2+}}^{°}$ value for Cr, Mn, Fe and Co are -0.41, +0.77 and +1.97 V respectively. For which one of these metals the change in oxidation state from +2 to +3 is easiest?

• Cr

• Mn

• Fe

• Co

The oxidation numbers of phosphorus in Ba(H₂PO₂) and xenon in Na₄XeO₆ respectively are

• +3 and +4

• +2 and +6

• +1 and +8

• -1 and -6