- B Phosphorus is the central atom in PCl 3 . A phosphorus atom needs 3
electrons to complete its valence shell. It gets 3 electrons by forming
covalent bonds with 3 chlorine atoms. The PCl 3 molecule has the
following structure:
Of the four electron pair sites around phosphorus, one is a
lone pair. This gives the PCl 3 molecule a trigonal pyramidal
shape.
- A First consider the neutralization that occurs between HBr and Ba(OH) 2.
2HBr + Ba(OH) 2 → BaBr 2 + 2H 2 O
Notice that for every 2 moles of HBr, only 1 mole of
Ba(OH) 2 is needed for neutralization. We have 0.1 liters (or
100 milliliters) of 0.2 M HBr. This means we have 0.1 liters
× 0.2 mole/liter, or 0.02 mole of HBr. We need 0.01 mole of
Ba(OH) 2 to neutralize 0.02 mole of HBr. Twenty-five
milliliters of 0.4 M Ba(OH) 2 (aq) has 0.025 liters × 0.4
mole/liter, or 0.01 mole of Ba(OH) 2.
- A A small Ka indicates a weak acid. That means statement III is false, and
therefore we can eliminate choices (D) and (E). It also means that most
HCN remains as intact molecules, as opposed to H+ and CN− ions, so
statement II is false. Process of elimination tells us that statement I must
be true. And it is: HCN H+ + CN−. Notice that the molar ratio of
H+ to CN− is 1:1.
- E Ionization energies get very large once we try to remove core electrons,
which are attracted more strongly to the nucleus than valence electrons.
So an atom with a very high second ionization energy would be expected
to have just 1 valence electron: The second electron to be removed from
such an atom would have to be a core electron. Among the choices, only
