Dick's Responses in WyzAnt Answers

The octet rule isn't really a reason why.  It's simply an observation as to what happens.  The full answer is rather complex, but it mainly amounts to two properties:  ionization energy (how much energy it takes to remove an electron) and electron affinity (the ability to gain electrons).

Simply put, alkali metals have relatively low ionization energies, so it is easy to remove electrons from the neutral atoms.  They have unfavorable electron affinities, so they don't like to gain electrons.

On the other side of the periodic table, halogens are the opposite.  They have favorable electron affinities, so it is favorable to gain electrons to form negative ions.  They have very high ionization energies, so it is very difficult to remove electrons.

Once an atom has lost electrons to form an octet, the outermost electrons are in a lower shell, closer to the positively charged nucleus, so are much harder to remove.  This is why they stop at the noble gas configuration.

The other end is a bit trickier, but once an electron has been added to a neutral atom, a negatively charged ion is formed.  Adding a negative to a negative is always unfavorable (think of bringing two north poles on a magnet together).  The reason it sometimes happens has to do with the very favorable formation of solid crystals and the efficiency of packing that results.  In any case, once the octet is obtained, not only are you adding a negative to a negative, you are trying to add it to a higher level shell, farther from the nucleus, so there is very little attraction holding onto the electron.

Now, looking back at your original question, I notice that you are asking two completely different questions.  The title of the question asks about covalent bonds, while the text asks about ionic bonds.

Correct answer:  None of the above

Probable expected answer:  benzene

As Daniel stated, a secondary carbon has two adjacent carbons (benzene is the only one that fits).

However, there is another aspect of the definition, in that the term is only used for tetrahedral (sp³) carbons.  Of the choices you were given, only the second has all tetrahedral atoms, but they are not all secondary.

The mass number is not the same as the mass of the atom.

The mass number is obtained by counting the number of protons and neutrons.

Each proton and neutron is approximately 1 u, but not exactly.  Also, in atoms, the total is not the sum of the parts.  A small amount of mass is converted to energy to keep the nuclear particles together.

In summary, the atomic mass is the actual mass of the atom, while the mass number is simply the number of nuclear particles present.

What, exactly, are you asking? Is what true? This is very convoluted.

There are errors in the calculations above:

line 2, the mass of an atom of Li is 6.94 u, but close enough

line 7, remove the "kg" from the end.

line 9, put parentheses around the denominator, otherwise you'll get an error

In the last statement, it looks like what you're saying is:

7 g = 6.022 x 10^23 atoms, which is true. 7 g is the molar mass of Li, which means that 7 g Li = 1 mol Li = 6.022 x 10^23 atoms Li.

The first answer is incorrect.  At the stoichiometric point, the concentration of the NH3 is almost zero (all of the NH3 has reacted with H+).  To a first approximation, it is zero.  All that's present is NH4+ (plus the NO3- spectator ions, which we'll ignore).  Since NH3 is a weak base, its conjugate (NH4+) is a weak acid, with a Ka of its own.

The reaction involved at this point is:

NH4+  <==> H+ + NH3

Ka = ([H+] * [NH3]) / [NH4+]

However, we're given Kb for NH3, not Ka for NH4+.  Fortunately, it's easy to calculate the Ka for NH4+

Ka (NH4+) = Kw / Kb (NH3) = 5.6 x 10^-10

Now, in order to find the pH, we need to find [H+] (we'll use the variable 'x' for simplicity).  Realize that whenever an H+ is formed, an NH3 molecule is also formed, so [NH3] also equals 'x'.  The [NH4+] was calculated correctly in the previous answer, and is 0.25 M

Thus, Kb = x^2 / 0.25 =  5.6 x 10^-10

x^2 = 0.25 * 5.6 x 10^-10 =1.4 x 10^-10

x = 1.18 x 10^-5

Remember that x stands for [H+], and pH = -log [H+]

pH = -log (1.18 x 10^-5) = 4.93

A couple of comments:

1) Always think back to the problem, to make sure your answer makes sense. At the equivalence point, all of the NH3 has been converted to NH4+.  Since NH3 is a weak base, NH4+ is a weak acid.  Since you have a solution of a weak acid, the solution must be acidic.  If the solution is acidic, the pH must be less than 7.

2) The point at which the pH = pKa, or pOH = pKb occurs at the half-equivalence point, (i.e., when enough titrant has been added to react with half of the acid or base present).  In the situation you describe, that would have happened when 150 mL of HNO3 had been added.

3) Remember when I said [NH3] was essentially zero?  Look above, and you'll see that it is actually equal to 'x'.  Since x = 1.18 x 10^-5, [NH3] is also equal to 1.18 x 10^-5 M (which is pretty close to zero compared to its initial value of 0.3 M)