state. The ground state of an atom is the state in which all the electrons in the atom are in
their lowest energy state. If any electron has absorbed energy and been promoted to a higher
energy orbital in the atom without actually leaving the atom, the atom is said to be in an
excited state. An atom can have any number of excited states depending on how many
electrons have been promoted and what orbitals they end up in. Usually, excited states are
unstable and the atom will release energy and return to the ground state. So which of these
choices explains the difference between these two definitions? Choice D: When electrons gain
energy and change which atomic orbital they’re in, they change their quantum numbers,
which in turn changes the electronic configuration around the nucleus.33 . D
This question deals with solubility constant and the common ion effect. The first step to
solving this problem is to express the solubility product constant, which is the ion product of
the saturated solution, as the product of the concentration of Mg2+ ion and the concentration
of the hydroxide ion squared. Second, you must determine the minimum concentration of
hydroxide necessary to precipitate the Mg(OH) 2 . From the equation given, you can solve for
the concentration of hydroxide, which is the square root of the Ksp divided by the
concentration of Mg2+. Substituting in the values provided in the question, you should find
that the concentration is equal to 10–3 mol/L. That means that the pOH of the solution will be
3; therefore, the pH must be 11.
34 . C
The random motion of a gas holds the most translational kinetic energy.
35 . D
This question asks you which of the choices will increase the rate at which ice melts in a closed
container. If you lower the temperature, you are tipping the equilibrium toward the ice, so B is
wrong. To deal with pressure changes, we must apply Le Châtelier’s principle, which says that
a system in equilibrium that is subject to stress will shift its equilibrium so as to relieve the
stress. If the pressure is lowered, as in choice C, the system will counteract the change in
pressure by shifting its equilibrium toward the phase that is less dense. In the case of water,
that is the ice. Remember that water has a strange property in that the solid form, ice, at zero
degrees Celsius is less dense than the liquid phase, water, at that temperature. Since the
reduction of pressure drives the systems to produce ice, choice C is incorrect. However, choice
D, an increase in pressure, will have the opposite effect and the water will be produced
preferentially, meaning that the ice is melting faster. Choices A and E both favor ice formation.