Re: My brain is fried and I need help

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From:"J. A. Kiernan" <>
To:"Jennings-Siena, Debbie" <>
Content-Type:TEXT/PLAIN; charset=US-ASCII

On Fri, 11 Feb 2000, Jennings-Siena, Debbie wrote:

> You can laugh at me later but I really need your help with this math
> question.   I need to make up a 0.2N solution of HCL.  Our HCL is 36.6 to
> 38% and the FW is 36.46.  Help please.

  You should not feel any embarrassment in asking this question.
  This sort of calculation is one of the easiest to make a mistake
  with. (THE easiest to bungle, IMHO, is making serial dilutions.)

  You need the following, from the bottle label of the
  concentrated acid: Percent assay (note that this is Wt/Wt,
  not the Wt/Vol that's usual when making a solution yourself);
  Density in g/ml (= specific gravity); Molecular weight (formula
  weight, FW). You also need to know the basicity of the acid.
  That's the number of hydrogens in the formula that can combine
  with a base. For HCl basicity=1. For H2SO4 basicity=2. 

  The formula for making one litre of diluted acid is:

              100  x  Mol Wt  x  Normality 
  V   =   --------------------------------------   
          Basicity  x  Percent Assay  x  Density

  V is the volume of conc. acid needed.

  For your question I have guessed that the density is 1.19, and
  we'll settle for 37% as the assay - a figure in the middle of
  the range you quoted.

              100 * 36.46 * 0.2
  V   =    -----------------------
                1 * 37 * 1.19

      =    16.56

  [ If you are reading this with some ghastly Windows email program,
   the above equations may not look very elegant. They were typed
   into a real email program (Pine), with a simple non-proportional
   font. ] 

  So add 16.6 ml of conc HCl to about 900 ml of water and then
  make up the volume to 1000 ml.

  For a monobasic acid like hydrochloric, 0.2N is the same as 0.2M.

  If you need only an approximately 0.2N HCl, then assume that
  the conc. acid is 12M and dilute it 60 times. If you multiply
  the V above by 60 you'll see that it's only 4 ml off the 1000.
  (Having checked that, I'm reasonably confident that I didn't
  make any mistakes in the above formulas!)

  Two other useful rules of thumb for histochemical work are 
  that glacial acetic acid is 17.4M and that 1N sodium hydroxide 
  (useful for adjusting pH of buffers) is a 4% solution.

 John A. Kiernan,
 Department of Anatomy & Cell Biology,
 The University of Western Ontario,
 LONDON,  Canada  N6A 5C1

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