Answer
(a) $8.4\times 10^{23}~photons$
(b) $40~molecules$
Work Step by Step
(a) We can find the number of photons as
$n=\frac{mL_f}{hf}$
We plug in the known values to obtain:
$n=\frac{(1.0Kg)(80)(4186)J/Kg}{(6.63\times 10^{-34}J.s)(6.0\times 10^{14}Hz)}$
$n=8.4\times 10^{23}photons$
(b) We can find the required number of molecules of $H_2O$ as follows:
$N_{H_2O}=\frac{hf}{(m_{H_2O})L_f}$
We plug in the known values to obtain:
$N_{H_2O}=\frac{(6.63\times 10^{-34})(6.0\times 10^{14})}{(3.0\times 10^{-26})(80.0)(4186J/Kg)}$
$N_{H_2O}=40~molecules$
