A solid metallic block weighing 5 kg has an initial temperature of 500°C. 40 kg of water initially at 25°C is contained in a perfectly insulated tank. The metallic block is brought into contact with water. Both of them come to equilibrium. Specific heat of block material is 0.4 kJ.kg^-1.K^-1. Ignoring the effect of expansion and contraction and also the heat capacity to tank, the total entropy change in kJ.kg^-1.K^-1 is

A. -1.87

B. 0

C. 1.26

D. 3.91

Answer: Option B

Solution(By Examveda Team)

By energy balance $$m{c_p}\left( {{T_i} - {T_f}} \right){\text{ of block}} = m{c_p}\left( {{T_f} - {T_i}} \right){\text{ of water}}$$
So, we will get $${T_f} = 303.16\,k$$
We know,
$$\eqalign{ & \Delta {S_{total}} = \Delta {S_{BLOCK}} + \Delta {S_{WATER}} \cr & \Rightarrow \Delta s = m{c_p}ln\frac{{{T_f}}}{{{T_I}}} + m{c_p}ln\frac{{{T_f}}}{{{T_i}}} \cr} $$
$$ \Rightarrow $$ on substitution $$w$$ will get $$\Delta s = - 1.869 + 3.118 = 1.26$$