Question #b98fe

In one liter of a one molar solution, the number of moles of the solute is

#(M)(V) = (1 (mol)/L)(1L) = 1 mol#

For #KCl#, there are one atom of each per formula unit, so there are one mole of each #K^+# and #Cl^-# in one liter of a #1M# solution of potassium chloride.

For #Mg(NO_3)_2#, there are two moles of #NO_3^-# per one mole of #Mg^(2+)#, so there is one mole of #Mg^(2+)# and two moles of #NO_3^-# in one liter of a #1M# solution of magnesium nitrate.

Both #KCl# and #Mg(NO_(3))2# are salts. Thus they completely ionize when dissolved in water (to yield #K^(+)#, #Cl^(-)#, #Mg^(+2)# and NO_(3)^(2-), respectively.)
The number of moles of ions produced from #KCl# dissolving:
#1 mol K^+ +1 mol Cl^(-)=2 mol (ions)#
The number of moles of ions generated from #Mg(NO_(3))2# dissolving:
#1 mol Mg^(2+) +2 mol(NO_(3)^(-))=3 mol (ions)#

However, if you take the reversible reaction into account: #Mg^(2+)+2(OH^-) rightleftharpoonsMg(OH)2#
#K_(sp)=5.61×10−12# for #Mg(OH)2#.
Let the number of #Mg^(2+)# converted in this reaction equals to #x#, using the RICE table,
#(1-x)*(10^(-7)-2x)=5.61×10^(−12)#, #x=4.99972*10^-8#.
This value is too small to be considered.
Therefore the moles of ions in #1L# of #1M# #Mg(NO_(3))_(2)# solution is #3 mol#.

(reference: https://en.wikipedia.org/wiki/Magnesium_hydroxide )
[What's the meaning of the ICE table.. as seen in this equilibrium solution?],(https://socratic.org/questions/what-s-the-meaning-if-ice-table-as-seen-in-this-equilibrium-solution).