Question

The dissolution of 1 mole of NaOH(s) in $100moleof{H}_{2}O\left(l\right)$ give rise to evolution of heat as -42.34 kJ. However, if1 mole of NaOH(s) is dissolved in $1000moleof{H}_{2}O\left(l\right)$ the heat given out is 42.76 kJ.

If the enthalpy change when $900moleof{H}_{2}O\left(l\right)$ are added to a solution containing 1 mole of NaOH(s) in $100moleof{H}_{2}O$ is $x$ $kJ$:

• A. $-0.42$
• B. $0.42$
• C. $0.24$
• D. None of these

Answer 1

The correct option is A $-0.42$

$\Delta H_{1}for1moleNaOHin100mole{H}_{2}O\left(l\right)=-42.34kJ$

$NaOH\left(s\right)+100H_{2}O⟶NaOH\left(100H_{2}O\right);\Delta H=-42.34kJ$ ... (i)

$NaOH\left(s\right)+1000H_{2}O⟶NaOH\left(1000H_{2}O\right);\Delta H=-42.76kJ$ ... (ii)

By eqs. [(ii) - (i)]

$NaOH\left(s\right)+900H_{2}O⟶NaOH\left(900H_{2}O\right);\Delta H=-0.42kJ$

Hence,option A is correct.