[malteser16]

Hello,

I need to use Mohr's titration to discover the amount of chlorides in a water sample using silver nitrate solution (0.025N) and potassium chromate as indicator.

the calculation is as follows;

Calculate the chloride ion concentration in the original sample, in mg/L as follows;

Chloride, mg/L = [(A-B) x N x 70906]/S

Where;
A = standard solution AgNO3 added in titrating the sample prepared, mL,
B = standard solution, AgNO3, added in titrating the blank, mL,
N = normality of standard AgNO3 solution, and,
S = original sample in the 50mL test specimen prepared, mL.

where did the conversion factor 70906 come from? what is it converting exactly? and how was it calculated?

Thanks for any help.

[Borek]

70906 is molar mass of Cl₂ or 2Cl^- (expressed in mg). IMHO if this formula is to be used for Cl^-, it should be 35453.

[malteser16]

Hello,

Thank you so much for that clarification, Yes I understand your point in using 35453, I am using the ASTM D512-04 method for chloride ion in water, Method B to be more precise.

Why do you think they have chosen 70906 instead of 35453? are the end formations Silver chromate and potassium chloride, or am I wrong?

Thanks for the help.
 

[Borek]

http://www.titrations.info/precipitation-titration-argentometry-chlorides-Mohr

No idea why 70906.

Formula provided is basically (titrant volume)*(titrant normality)*(molar mass in mg)/(sample volume) - that's direct application of 1:1 stoichiometry, C=n/V and n=m/M (Concentration, number of moles, Volume, mass, Molar mass). Normality in this case equals molarity.

[xoto]

why not forget about already made formulae (if you are having problems) and solve from scratch, using balanced equations and mole ratios?