The only possibility of heat flow in a thermos flask is through its cork which is `75cm^2` in area and 5 cm thick its thermal conductivity is `0.0075 cal//cm-s-^@C`. The outside temperature is `40^@C` and latent heat of ice is `80 cal//g`. Time taken by 500 g of ice at `0^@C` in the flask to melt into water at `0^@C` is `

Options

A. 2.47 h
B. 4.27 h
C. 7.42 h
D. 4.72 h

Answer

A

Explanation

`mL=(KaDeltathetat)/(Deltax)`
`implies=500xx80=(0.0075xx75xx(40-0)t)/(5)`
`impliest=8.9xx10^3s=2.47h`