Silicon, which makes up about 25 % of the Earth’s crust by mass, is widely used in the modern electronics industry. It has three naturally occurring isotopes, Si-28, Si-29, and Si-30. Calculate the atomic mass of silicon.
Isotope Isotopic Mass % Abundanc
Si-28 27.976927 92.23
Si-29 28.976495 4.67
Si-30 29.973770 3.10

Two nitro no2 groups are chemically bonded to a patch of surface. they can't move to another location on the surface, but they can rotate (see sketch at right). it turns out that the amount of rotational kinetic energy each no2 group can have is required to be a multiple of ε, where =ε×1.010−24 j. in other words, each no2 group could have ε of rotational kinetic energy, or 2ε, or 3ε, and so forth — but it cannot have just any old amount of rotational kinetic energy. suppose the total rotational kinetic energy in this system is initially known to be 32ε. then, some heat is removed from the system, and the total rotational kinetic energy falls to 18ε. calculate the change in entropy. round your answer to 3 significant digits, and be sure it has the correct unit symbol.

What is the value of the standard enthalpy of formation of an element in its most stable form?

Match each term to its description. match term definition excess reactant a) reactant that can produce a lesser amount of the product limiting reactant b) amount of product predicted to be produced by the given reactants theoretical yield c) reactant that can produce more of the product