Ask A Scientist Chemistry Archive

name         Aa
status       student
grade        6-8
location     IN

Question -   What is the difference between acid strength and acid 
concentration?
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As you know, an acid is a compound that produces H+ ( a proton) in 
solution. How easily this proton is released from the compound is the 
indication of the acid's strength. How many protons there are in solution 
is the indication of the solution's acidity. Thus, HCl - which readily 
gives up its protons will have a high acid strength - even if there is 
only a few HCl molecules in the solution. On the other hand, HF does not 
easily give up its protons - which means it is a weak acid (not very high 
acid strength) even though there may be a lot of it in solution (which 
would give it a high acidity).

Greg (Roberto Gregorius)
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"Acid Concentration" is well defined.
It means: how many acid molecules per unit volume in the liquid.
   Or more particularly, how many H+ ions are donated by the molecules in 
 a unit volume.
That because some acid molecules have 2 or 3 H+ ion to donate.

Acid Strength can mean a couple of different things.
   a) Is that species of molecule strongly acidic?
         When comparing acid A to acid B, which one donates its H+ ion 
 more easily.
         HCl in water becomes H+ and Cl- right away,
           so the H+ is just wandering around in that water, looking for 
 some damage to do.
         So we say "HCl is a strong acid".
         We also say the Acetic Acid in vinegar is a weaker acid.
          Its molecules H(C2O2H3) try to keep their H's until somebody 
 wants it more than they do.
         The H+ concentration is less, so the acid does less "damage" or 
 reacting with things.

  b) Is that solution strongly acidic?
         A solution is a bucket of water with some concentration of 
 chemicals dissolved in it.
         A strongly acidic solution must have a high instantaneous 
 concentration of free H+ ions,
             regardless of the concentration of other chemicals.
         A concentrated solution of a weak acid molecule is still a weakly 
 acidic solution.

There are two independent measures of acidity:
   - concentration of the supposedly acidic molecule,
             even if it's keeping 99.9% of it's H+'s to itself.
   - concentration of free H+ ions.
          The "-log(base10)" of this concentration is called "pH".
          pH 4  means 10^-4 molar [H+], which is a weak acid solution
          pH 1 means 0.1 molar [H+], which is a fairly strong acid solution.
You need to pin down both to understand the solution well.


A pH 4 weak acid solution could be
     -   a concentrated solution of a weak acid,
     -   or a 10,000 times diluted solution of a strong acid molecule.
A strong acid solution with pH=0 can only be
         a concentrated solution of a strongly acidic molecule.

In grade 6-8 you may not have heard:   a mole is 6.02 x 10^23 molecules.
It is how many H atoms it takes for a mass of one gram.
All the other atoms and molecules are heavier,
but we still need to handle them by count, not by mass.
"Molarity" is concentration in moles per Liter.

Jim Swenson
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The terms "strength" and "concentration" are frequently confused when
used in the context of (acids / bases) that dissociate into ions to a
limited extent when dissolved in water. The confusion is caused by the
fact that in common usage "strength" and "concentration" are synonyms.
That is not the case when referring to "weak" (acids / bases) in a
chemical context. In chemistry, "concentration" has its common definition:
The amount (grams, mols, pounds, etc.) of a substance dissolved in a
specified amount of solvent (in this case, water).
   However, the terms "strong" or "weak" refer to the relative amount of
substance present in ionic form compared to the amount of the same substance
present in molecular form. An acid (or base) is "strong" if it is entirely,
or almost entirely, present in solution in the form of cations (+) and
anions (-). Thus HCl (hydrochloric acid) is a "strong" acid because when
dissolved in water the dissolved entities are essentially 100% (H[+1]) and
(Cl[-1]) regardless of the concentration 0.0001 mols/liter or 1.0
mols/liter.     In contrast, acetic acid (common vinegar, call it HAc for
short) is only dissociated slightly into ions
(H[+1]) and (Ac[-1]) when dissolved in water. Most of the acetic acid
dissolved in water is present in the electrically neutral unionized
molecular form (HAc). It is possible to measure the extent to which the
"weak" acid is ionized. The extent of ionization is usually expressed as a
ratio of concentrations:
K = (H+) x (Ac-) / (HA) where the concentrations are usually expressed in
units of (mols/liter). To a good approximation the value of 'K' is constant
regardless of the concentration of acetic acid, that is the total amount of
acetic acid present, regardless of whether it is present as ions or as
neutral molecules. The relative amounts can change, but not the expression
for 'K':     K = (H+) x (Ac-) / (HA)

Vince Calder
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