- A The first quantum number refers to the energy level, so it would be 3. A
d-orbital is represented by a 2, giving us the second number. For the third
number, the five possible numerical representations for a d-orbital
orientation are −2, −1, 0, 1, or 2. The final quantum number, representing
the spin, could be either or −.
- E First, according to the periodic table, Cr has 24 electrons. We can
eliminate choices (B) and (C) because those configurations have 25
electrons. Choice (A) is wrong because the 3s subshell is already
accounted for in the [Ar] core—i.e., [Ar] stands for 1s^22 s^22 p^63 s^23 p^6.
Now, according to the Aufbau principle, we completely fill subshells
before moving up to the next higher one. So the best answer would
appear to be choice (D). However, remember that completely half-filled
and filled d subshells bestow extra stabilization to an atom. Therefore, Cr
and Cu actually violate the Aufbau principle and promote a 4s electron to
become [Ar] 4s^13 d^5 and [Ar] 4s^13 d^10 , respectively. Choice (E) is the
correct ground state configuration for Cr.
- D At 25°C, pH + pOH = 14 for any solution. Therefore, if the pH is 10 for
this solution, the pOH is 14 − 10 = 4. Taking the negative antilog of 4
gives 10−4, choice (D).
- C Chromium is a transition metal, so the charge on the cation in this
compound must be specified. Carbonate has a charge of negative two,
and to balance it, the chromium must have a charge of positive two.
- D First, find the number of moles of Br−.
molarity = moles/volume or moles = molarity × volume
= 0.1 M × 0.5 L
= 0.05 moles
Then figure out how much 0.05 moles of Br− weighs.
