[Pepealej]

Hi, I've a doubt with conjugate acids and bases.

We know that [tex]K_a \ (acid) * K_b \ (conjugate \ base)=K_w[/tex] and viceversa, with which we se:

(1) Strong acid --- Weak conjugate base
(2) Weak acid --- Strong conjugate base

But in General Chemistry. Petrucci (10th ed. Page 723) they say that the conjugate of something weak is always something weak. I'm confused.

¿How can I determine the strenght of a species then?

Thanks

[Borek]

7+7=14 - both weak.

I suppose what they mean is that the rule should be stated in terms of very strong, weak, very weak - then conjugate of very strong is very weak, but conjugate of weak is weak. Handwavy, but more accurate then the rule you quoted.

[Dan]

The problem here is that "weak" is a comparative term and in this case a reference point is not given. It is meaningless to call something a "weak" without definition or context.

Try not to get tied up in the language, your understanding in points (1) and (2) is correct.

As the acid gets stronger, the conjugate base gets weaker - I think what is referred to as "weak" in your book is the the middle ground where both are "moderate". An example might be H₂S (pKa = 7).

[Pepealej]

So, if I have a weak base, I will get a strong acid (strength that depends on the weakness of the base), won't I?

Thanks

[Babcock_Hall]

My preferred way of thinking about it is to think mainly in terms of comparisons.  Suppose that I have two acids, HA and HB.  If HA is a stronger acid than HB, then B^- is a stronger base than A^- is.  I agree with the implication of the previous comments, that the language can mislead the unwary.  However, we can flesh out Petrucci's comment, as Borek has suggested.  Suppose we think about phosphate monoanion, which has a pK of about 7.2 at low ionic strength.  It is not a strong acid, in the sense that would tend to ionize very little in water.  Its conjugate base, the phosphate dianion, would also not tend to ionize much in water; in other words, it is a weak base.

[AWK]

So, if I have a weak base, I will get a strong acid (strength that depends on the weakness of the base), won't I?

Thanks

For corresponding pKa and pK_b their sum is always 14 at standard conditions.
The border between strong and weak is usually set about -1 in logarithmic scale (pK_a, pK_b)
Hence pK_a ...-3, -2 , -1 - strong acids,
Pk_a over -1 - weak acids
at Pk_a =15 the value of pK_b=-1 and this is a beginning pK_b for strong bases.
Between pK_a -1(pK_b 15) and pK_a 15 (pK_b -1) both acids and bases are weak (sometimes terms medium weak or medium strong are used for these pK values to remove ambiguity)