Many chemical reactions are brought about by light: think of photosynthesis for the creation of carbohydrates, tanning caused by the ultraviolet component of solar radiation, and the dramatic molecular events that take place in the upper atmosphere. To understand these processes, chemists need to know how much energy is transferred to a molecule when a photon collides with it. What is

(a) The energy of a single photon of blue light of frequency 6.4 * 10¹⁴ Hz;

(b) The energy per mole of photons of the same frequency?

ANTICIPATE From Table 1A.1 you can see that the energy of a photon of blue light should be about 4 * 10^–19 J.

PLAN (a) Use Eq. 2 to find the energy of light of a given frequency. (b) Multiply the energy of one photon by the number of photons per mole, which is Avogadro’s constant (Fundamentals E).

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E = hv (2)