Question

One mole of anhydrous $MgC{l}_2$ dissolve in water and liberates $25$ cal/mol of heat. $\Delta H_{hydration}$ of $MgC{l}_2=-30$ cal/mol. Heat of dissolution of $MgC{l}_2\cdot H_{2}O$ is:

• A. $+5$ cal/mol
• B. $-5$ cal/mol
• C. $55$ cal/mol
• D. $-55$ cal/mol

Answer 1

The correct option is B $-5$ cal/mol

According to the problem:

Given that:

Let us consider the heat is liberated when $MgC{l}_2$ dissolved in water is $25\frac{cal}{mol}$.

Heat of ionisation is $25\frac{cal}{mol}$

Heat of hydration is $-30\frac{cal}{mol}$

Therefore,

Heat of solution is Heat of ionisation plus Heat of hydration

Implies that

$=25+(-30)$

Implies that

$=(-5)\frac{cal}{mol}$