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Dilution of concentrated acids: formula etc.
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Dilution of concentrated acids: formula etc.


If I want to make a 1N solution of, for example, hydrochloric acid how do I convert the liquid, concentrated HCl into a gram value. The bottle of concentrated HCl says it is a 35-36% solution.


This applies to dilution of all concentrated acids (and also to strong ammonia (ammonium hydroxide) solutions.

The percentage on the label is weight/weight, not weight/volume, so you have to take into account the density of the concentrated acid.

The formula for making one litre of a particular normality, N, is:
           100 X M X N
       V = ------------
            B X P X D
where V is the volume of concentrated acid needed, M is is its molecular weight, N is the desired normality, B is the basicity (1 for most common acids; 2 for sulphuric; 3 for phosphoric; 1 for ammonia), P is the percentage by weight in the concentrated acid - the figure on the label, and D is the density of the conc. acid (specific gravity) in grams per ml.

No, I didn't work it out myself; it's from Lange's Handbook of Chemistry.

If the dilution doesn't need to be very precise, you can assume the following normalities for common concentrated acids:

    Hydrochloric (36%) 12N
    Nitric (71%) 16N
    Sulphuric (96%) 36N (= 18M)
    Acetic (99%+) 17.4N
    Formic (90%) 23.4N

So to make approximately 0.5N hydrochloric acid, you dilute the conc. HCl 24 times. To make a litre, you'd measure 42 ml of the conc. acid (because 1000/24=41.7) and add it to about 800 ml of water. Stir, and make up to a final volume of 1000 ml.

Remember to pour the acid slowly into the water, especially sulphuric acid, which generates a lot of heat when mixed with water.

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

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