Explain the law of conservation of mass and how it applies to balancing chemical equations in chemistry

If you chip a tooth, most likely you will go to the dentist to have the missing material filled in. currently the material used to fill in teeth is a polymer that is flexible when put in, yet is hardened to the strength of a tooth after irradiation with blue light at a wavelength of 461 nm. what is the energy in joules for a photon of light at this wavelength?

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Suppose you wanted to make 100 grams of water. what is the molar mass of water (h2o)?