Question

Doubt in Law of equivalents?

According to law of equivalents, number of gram equivalents of all reactants (none of them in excess) = number of gram equivalents of all products if all reactants undergo the reaction.

Are there any conditions/exceptions under which the law does not hold?

I noticed a difference in a mixture solution. The
equivalents add up.

For example this problem: When a mixture of (NaBr) and (NaCl) is repeatedly digested with (H_2SO_4, all the halogens are expelled and (Na_2SO_4) is formed quantitatively. With a particular mixture, it was found that the weight of (Na_2SO_4) obtained was precisely the same as the weight of (NaBr,NaCl) mixture taken. Calculate the ratio of (NaCl) and (NaBr) in the mixture.

I could comfortably solve this question using mole concept and POAC but the solution I have uses equivalent concept and I didn't understand this step:
Number of equivalents of (NaCl) + Number of equivalents of (NaBr) = Number of equivalents of (Na_2SO_4).

Answer 1

Well mass is conserved in EVERY chemical reaction....

Explanation:

And this has been the observation of centuries of research and endeavour...`"there ain't no such thing as a free lunch"`, and `"garbage out equals garbage in..."`

And here, you certainly got a FINITE mass of sodium ion... And you propose to isolate this as sodium sulfate. And so because the molar quantity of `NaCl` and `NaBr` is definite, and so is the mass of sodium....the molar quantity of `Na_2SO_4` is also definite, and so is its mass, which you attempt to collect.

And the molar quantity of `Na_2SO_4` MUST EQUAL the SUMS of the molar quantities of `NaCl` and `NaBr`, because these individual salts are the only source of sodium ion. Do you see from where I am coming?

Anyway, if you gots an objection, or a query, or a speculation, feel free to post it, and someone will attempt to muddy the waters...