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Question: One mole of anhydrous MgCl2 dissolves in water and librates 25 cal/mol of heat. \(\Delta\)Hhydration...

One mole of anhydrous MgCl2 dissolves in water and librates 25 cal/mol of heat. Δ\DeltaHhydration of MgCl2 = –30 cal/mol. Heat of dissolution of MgCl2.H2O is

A

+5 cal/mol

B

–5 cal/mol

C

55 cal/mol

D

–55 cal/mol

Answer

+5 cal/mol

Explanation

Solution

MgCl2.H2O \longrightarrow MgCl2(aq) Δ\DeltaH1 = ?

MgCl2(aq) \longrightarrow Mg2+(aq) + 2Cl–(aq) Δ\DeltaH2 = –30 cal/mol

MgCl2 (s) \longrightarrow Mg2+(aq) + 2Cl– (aq) Δ\DeltaH3 = – 25 cal/mol

Δ\DeltaH1 + Δ\DeltaH2 = Δ\DeltaH3

Δ\DeltaH1 = Δ\DeltaH3 – Δ\DeltaH2 = –25 + 30 = + 5 cal/mol.