Question

Microwaves are used to heat food in microwave ovens. The microwave radiation is absorbed by moisture in the food. This heats the water, and as the water becomes hot, so does the food. How many photons having a wavelength of 3.00 mm would have to be absorbed by 2.54 g of water to raise its temperature by 1.00 °C?

Answer 1

How much heat (energy) do you need? That's q = [mass H2O x specific heat H2O x (Tfinal-Tinitial)
Solve for q in joules.

How much heat do you have per photon?
That's E(in joules) = hc/wavelength.
Solve for E
Then E per photon x # photons = q
Solve for # photons.