Greetings knowitall22,
Thank you for your question! Although I've taught chemistry, I
managed to learn a few things. At first, I had no clue (or have
forgotten) what muriatic acid was, or what '20 Baume' might be, but as
usual Google was able to help.
Muriatic acid is a traditional name for hydrochloric acid (HCl)
http://webserver.lemoyne.edu/faculty/giunta/archemm.html
Baume is a method of measuring the specific gravity, or density, of a
chemical:
http://www.potters.org/subject23952.htm
The given formula yields a value of the specific gravity of
145/(145-20) = 1.16
So a mL of the solution has a mass of 1.16g
Information on commercial preparations of 20 Baume muriatic acid shows
that the percentage of HCl in the solution (by weight) is 31%:
http://www.potters.org/subject34114.htm
Thus to translate the information into something I'm a bit more
familiar with, we want to know what quantity of a 31% solution of HCl
will neutralize 1000 mL of NaOH at a ph of 8.
-------------------------------
HCl and NaOH are respectively a strong acid and a strong base. Thus,
they both completely dissociate into ions in solution, making the
calculation much easier. A pH of 8 means that the pOH is 6 (since pH
+ pOH = 14)
The molar concentration of hydroxide ions [OH-] is given by 10^-pOH =
1 * 10^-6 M.
In 1000 mL (one liter) of this solution, there are 1 * 10^-6 moles of
[OH-] ions. To neutralize them [i.e. bring the pH to 7] we would need
to add the same amount of H+ ions, which will come from the
hydrochloric (muriatic) acid. Using what we know of the muriatic acid
solution, we can figure out the necessary volume to add in a chemistry
calculation with lots of factors to be multiplied:
1 * 10^-6 moles H+ * 1 mole HCl/1 mole H+ * 36.5 g HCl/mol HCl *
* 100g solution/31g HCl * 1mL solution/1.16g solution
The last two conversions come from the weight percentage of the acid
and its specific gravity. All the units cancel out except for mL of
solution, giving the result: 1.02 * 10^-4 mL
Not much, but the solution was already close to neutral, and the acid
is quite strong.
------------
To change the pH to 6, we would need to bring the [H+] concentration
to 1 * 10^-6 M, so we would need twice as much. The first 10^-6
neutralizes the base up to pH 7, the second 10^-6 brings the pH up to
pH 6.
To go from the original solution to pH 6 would require 2.03 * 10^-4 mL
[There are some subtleties here as well, due to the fact that there is
always an equilibrium between H+ and OH- in solution. You can't just
'get rid of' all of the 10^-6 of OH- by adding the same amount of H+.
However, the error introduced by my simplifying assumption is only
about 1%, I believe. The uncertainty of some of the other known
quantities, such as the specific gravity and weight percentage of the
muriatic acid are of the same size, so the assumption should not
affect the accuracy of the answer.
Also, after adding the acid, the volume is no longer exactly 1000 mL,
but the difference is so tiny that again it will have no effect on the
answer.]
Other sites:
http://faculty.washington.edu/varani/chem-162-website/Lecture_14_gv.pdf
This lecture discusses many of the topics covered in this answer.
Note particularly the section on Strong Acid-Strong Base titrations
Qualitative. In the discussion of equivalence, the pH becomes 7 when
the same amount of acid and base are added together. Note also how
steep that curve is in the graph near ph=7. It illustrates why your
calculation requires so little acid to change the pH from 8 to 7 or 6.
Near 7, small amounts of acid can have large effects on pH.
http://antoine.frostburg.edu/chem/senese/101/acidbase/faq.shtml
This FAQ offers a great deal of information on acids and bases. The
encapsulating site has excellent resources for all aspects of
chemistry.
Search strategies used:
muriatic acid
baume specific gravity
"20 Baume" muriatic
neutralization acid calculation strong
"online chemistry text"
I hope my response has answered your question without either
mystifying you or boring you with things you are already familiar
with. Let me know if I can make anything in my answer clearer for
you.
Best Regards,
--Azathoth |
Clarification of Answer by
azathoth-ga
on
20 May 2003 12:49 PDT
1.02 * 10^-4 mL = .000102 mL
2.03 * 10^-4 mL = .000203 mL
Yes, the numbers do seem ridiculously small! However, I believe they
are correct. The muriatic acid is very strong. With a specific
gravity of 1.16, a liter has a mass of 1160 grams, of which 31% (360
g) is HCl. This is about ten moles, so the acid is roughly 10 molar
in concentration.
To neutralize the base, we only need to add a millionth of a mole
(10^-6 moles). If each liter has ten moles of HCl, then we only need
to add one ten-millionth of a liter, or one ten-thousandth of a mL ~
.0001 mL
Regards,
--Azathoth
|
