MadSci Network: Chemistry

Re: Normal vs. % HCL in a solution

Area: Chemistry
Posted By: John Letourneau, Lab Technician, Canadian Forestry Service
Date: Thu Jul 3 12:57:39 1997
Area of science: Chemistry
ID: 867719559.Ch
	In chemical terms, concentrations or strengths of solutions are 
generally stated in one of two units of measure.  Solutions are sometimes 
stated in molar quantities (eg; 6M solution) or as a percentage.  
Percentage, sometimes listed as equivalence, is usually stated as w/w 
(weight of solute / weight of solution) so if you have a 50% solution of 
HCl in water, half of it's weight is HCl and half of it is water.  Molar 
quantities are a little more complicated which explains why percentages are 
more often used in industrial applications.
	 The molarity of a solution is expressed in mol / litre so if you 
have a solution that is 6 M (6 mol / litre) there is 6 mol of solute (HCl 
in your case) in every litre of solution.  Obviously, the mol unit here is 
of no use to you so you will need to be able to convert from mol to grams. 
 The molecular weight of a molecule is given in units of grams / mole and 
it is this number which is used in the conversion.  The molecular weight; 
M.W. or formua weight F.W. (the two are synonymous) is either listed on the 
container, can be deduced given the molecular formula using the periodic 
table of elements or you can look it up in most chemical supply catalogs 
under the appropriate chemical.
	The molecular weight of HCl for instance is 36.461 g/mole which 
means that for every mole of HCl there is 36.461 grams of HCL.  Therefore 
if you have a 6 mol/litre solution there is 6x36.461grams of HCl in that 
litre or 218.776 g/litre.  If you convert litres to grams (knowing that the 
density of water is 1 g/ml) you get 218.771 g of HCl/1000 g of solution or 
~22% HCl in water.  For more concentrated solutions the density of the 
solution may change and you must substitute this desity into the above 
calculation.  You can convert the other way (from % to molarity) by 
dividing the number of grams of solute/litre (in your case HCl) by its 
molecular mass giving you the number of moles/litre.  So, using your 50% 
solution as an example it would be a 13.7M solution of HCl in water.
	Normality and molarity are closely related and have to do with the 
concentration of acidic hydrogens in solution rather than acid molecules.  
For HCl solutions molarity and normality are equal because each molecule 
has only one acidic hydrogen atom.  For other acids which have multiple 
acidic hydrogens such as sluphuric acid wich has 2 and phosphoric acid 
which has 3 acidic hydrogens simply multiply the molarity by the number of 
acidic hydrogens to get the normality.
	In case this calculation is a little too complicated,  many 
suppliers list both the molarity and the pecent by weight on the container 
one of which will be in large bold print and the other will be in fine 
print (often somwhere else on the label).
	I hope that this has been helpful.  If you find all of this a 
little hard to wade through I'll give you a little equation you can use to 
do the conversion without worrying about the theory.

%by weight ={(Molarity x Molecular Weight)/(density x 1000 mL/litre)}x 100%

Molarity =(% by weight x density x 1000 mL/litre)/(Molecular weight x 100%)

	For the calculation to work properly the units of measure must be 
as follows;

Molecular weight - grams/mol (36.461 g/mol for HCl)
Molarity - mol/litre
Density - g/millilitre (Density may be listed as specific gravity)

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