3 N means : normality, the concentration of protons (hydrogen ions) in the solution is 3 mol / L

what is a mol ? 1 mol = 6.022

^{.} 10

^{23} molecules.

http://gemini.tntech.edu/~tfurtsch/scihist/avogadro.htm

Now we need to find out, how many protons HCl donates to the solution. HCl

^{__}> H

^{+} + Cl

^{-}As you can see 1 mol of hydrogen chloride gives 1 mol of protons in solution (dissociation). Therefore a 3N solution is 3M. H

_{2}SO

_{4} gives 2 protons, therefore a 3N solution of sulfuric acid would be 1.5M.

What does 3M mean. M is for molarity and specifies the concentration of a compound by mol / L. Just like normality but now it is not restricted to only protons (H

^{+}) anymore.

How to prepare a 3M soln. of HCl ?

What we need to find out is how much of a solution, of which is known that 36% of the weight are HCl, needs to be mixed with water to leave a concentration of 3 mol / L.

We've got the weight m , we need the amount of substance n.

M = m / n ;; molecular weight = mass / amount of substance. The question is how many grams do 6.022

^{.} 10

^{23} molecules (1 mol) weight ?

If you put amount of substance as 1, the molecular weight can be calculated from the atomic weights of its constituents, found in the periodic table of the elements.

H has an atomic weight of 1.0079 u

Cl has an atomic weight of 35.453 u

http://en.wikipedia.org/wiki/Atomic_weight

So one molecule of HCl weights 36,4609 u and thus one mol = 36,4609 g

Yeah, back to the solution we wanted to prepare.

3 mols will weight 109.3827 g. This much is in (109.3827 g / 36)

^{.} 100 = 303.84 g of a 36% HCl (the strength of concentrated HCl). To get the volume of those 303.84 g we need the density (which is dependant on temperature and usually given for 20°C if not specified differently). That is 1.18 g/mL.

http://ptcl.chem.ox.ac.uk/MSDS/acidsbases.html

.

So the volume will be 303.84 g / 1.18

^{g}/

_{mL} = 257,49 mL.

We're almost there. Since we want a concentration of 3 mol / L those 257,49 mL need to be mixed with water, until a total volume of 1000 mL is reached.

BTW: this also works exactly the same way with bases like NaOH, where N (normality) would specify the concentration of hydroxide ions (OH

^{-}) in the solution as mol / L.

mixing 15mL 36% HCl with 45mL should give a 2,913 N solution, with 38% HCl it would be

3,075 N. So it sounds about right.

Hope that helped anyone...

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**psyloxy**--