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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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