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Start by multiplying the grams of ice by the enthalpy of fusion in order to get the total energy required (in J) to melt the ice. Then substitute the given wavelength into the equation "E=(hc)/lambda" in order to find the energy per photon. Remember to convert nanometers to meters when using this equation. Then, you will have the total energy needed to melt the ice (J) and the energy (J) per photon. Divide the total energy by the energy per photon, and the result is the number of photons needed to have enough energy to melt the ice.
i started by multiplying the enthalpy of the fusion of water by the number of grams of ice. (333.6J/g * 459g = 153122.4J) you do this so you know the amount of energy required to melt that mass of ice. then i found the energy of one photon with the wavelength of 873 nm using the equations c=lamda(v) and E=h(v). once you solve for the energy of one photon, you divide the total energy (153122.4J) by the energy of one photon, which you have to solve for. then you end up with the number of photons. hope that helps :)
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