In which of the following cells, `EMF` is greater than `E^(c-)._(cell)`?

Options

A. `Pt,H_(2)(g)|H^(o+)(pH=5)||H^(o+)(pH=3)||H_(2)(g),Pt`
B. `Zn(s)|Zn^(2+)(0.2M)||Cu^(2+)(0.1M)|Cu(s)`
C. `Cr(s)|Cr^(3+)(0.1M)||Cu^(2+)(0.2M)|Cu(s)`
D. `Pt,H_(2)(g)|H^(o+)(pH=4)||H^(o+)(pH=6)|H_(2)(g)|Pt`

Answer

a

Explanation

`a.` Use `: E_(cell)=E^(c-)._(cell)-(0.059)/(n_(cell))log Q_(cell)`
For `E_(cell)gt E^(c-)._(cell),Q_(cell)lt1(i.e., log Q_(cell)lt0)`
`a.` `H_(2)(1 atm)+2H^(o+)(10^(-3)M) rarr 2H^(o+)(10^(-5)M)+H_(2)(1atm)`
`Q_(cell)=((10^(-5))^(2)xx1)/(1xx(10^(-3))^(2))=10^(-4)lt1`
`b.Zn(s)+Cu^(2+)(0.1M) rarr Zn^(2+)(0.2M)+Cu(s)`
`Q_(cell)([Zn^(2+)])/([Cu^(2+)])=(0.2)/(0.1)=2gt1`
`c.` `2Cr(s)+3Cu^(2+)(0.2M) rarr 2Cr^(3+)(0.1M) +3Cu(s)`
`Q_(cell) ([Cr^(3+)]^(2))/([Cu^(2+)]^(3))=((0.1)^(2))/((0.2)^(3))gt1`
`d. H_(2)(1atm)+2H^(o+)(10^(-6)M)rarr 2H^(o+)(10^(-4)M)+H_(2)(1atm)`
`Q_(cell)=((10^(-4))^(2)xx1)/(1xx(10^(-6))^(2))=10^(4)gt1`