water loss through transpiration?

Question

If a leaf is to maintain a temperature of 50°C (reasonable for a leaf), it must lose 250 W/m2 by transpiration (evaporative heat loss). Note that the leaf also loses heat by radiation, but we will neglect this. How much water is lost after 1 h through transpiration only? The area of the leaf is 0.006 m2.

Grams.

i thought it was 16.2 but it wasnt right

Answer 1

Energy loss due to transpiration is

Q=Pd A t=
Q=250 x 0.006 x 3600=5400J= 5.4 kJ

Q= m Ce
m -mass of water
Ce - latent heat of evaporation
Ce= 2258 kJ/kg
m= Q/Ce=5.400 /2258=0.0024kg or 2.4 gm

Note the specific heat of water is 4.186 kJ/(kg K)
We can ignore the amount of energy it takes to heat that amount of water to the boiling point.
[ Q= m Cp(T2-T1)
Q= 0.0024 x 4.186(100-50)=0.5 J see?]