All the salts below will be neutral to litmus paper when dissolved in water except? A. CuCl2 B. NaCl C. KCl D. Na2So4

Copper ions #"Cu"^(2+)# from copper chloride #"CuCl"_2# undergoes hydrolysis when dissolved in water to produce a slightly acidic solution.

#"CuCl"_2(s) to "Cu"^(2+)(aq) + 2 color(white)(l) "Cl"^(-)(aq)#

#color(blue)(ul(color(black)("Cu"^(2+)(aq)))) + "H"_2"O" (l) rightleftharpoons "Cu"("OH") ^(-)(aq) + color(red)(ul(color(black)("H"^(+)(aq))))#
#color(grey)("Hydrolysis of Cu"^(2+))#

Hydrolysis alters the #"pH"# of the solution. However, this process is possible only if one the ions of the salt is the conjugate base or acid of a weak acid or base. For example,

• #"Cu"^(2+)# is the conjugate #color(red)(ul(color(black)("acid")))# of the #color(blue)(ul(color(black)("base")))# #"Cu(OH)"^(-)#, a weak base. Thus it undergoes hydrolysis.
• As the hydroxide of potassium, an alkali metal, #"KOH"# (the conjugate of #"K"^(+)#) is a strong base. So is the case for #"Na"^(+)#.
• #"Cl"^(-)# is the conjugate #color(blue)(ul(color(black)("base")))# of the strong #color(red)(ul(color(black)("acid")))# #"HCl"#. It doesn't undergo hydrolysis.

#"H" stackrel(+6)("S")"O"_4^(-)# (the conjugate of #"SO"_4^(2-)#,) is a strong acid.

Solutions of #"NaCl"#, #"KOH"#, and #"Na"_2 "SO"_4# are therefore all supposed to be neutral since all their ions are conjugates of strong acids or bases meaning that none of them is capable of undergoing hydrolysis.

We could write the acid base reaction of the aquated copper(II) ion..

#[Cu(OH_2)_6]^(2+) + H_2O rightleftharpoons [Cu(OH_2)_5(OH)]^+ + H_3O^+#

#K_a =3xx10^-8#...data are from a text.
And so we work from a solultion that is formally #0.1*mol*L^-1# in copper ion...

#K_a=([ Cu(OH_2)_5(OH)^+][H_3O^+])/([Cu(OH_2)_6]^(2+)]=x^2/(0.1-x)=3xx10^-8#

And so #x_1=sqrt(3.0xx10^-9)=5.46xx10^-5*mol*L^-1#

#x_2=5.48xx10^-5*mol*L^-1#

#pH=-log_10[H_3O^+]=5.26#..the solution is slightly acidic...