Is law of equivalence valid for partial displacement reactions?

【I found this in my textbooks: According to the law of equivalence, whenever two substances react, the equivalents of one will be equal to the equivalents of other and the equivalents of any product will also be equal to that of the reactant.

If I consider this reaction, then the law of equivalence holds good: $$\ce{H2SO4 + 2NaOH -> Na2SO4 + 2H2O}$$ Here, number of gram equivalents of each of the reactants and products are equal.

But, if I consider this reaction: $$\ce{H2SO4 + NaOH -> NaHSO4 + H2O}$$ Here, 2 equivalents of $\ce{H2SO4}$ react with 1 equivalent of $\ce{NaOH}$ to produce 1 equivalent of $\ce{NaHSO4}$ and 1 equivalent of $\ce{H2O}$ .

Here, the law of equivalence does not hold true.

Similarly, $$\ce{HC#CH ->[1eq of NaNH2] NaC#CH}$$ The law of equivalence does not hold true for this.

Please explain the limitations of law of equivalence, if any.

  • 2
    $\begingroup$ "number equivalents" depends on reaction, that's one of reasons they are obsolete and I wonder why they were introduced in first place. $\endgroup$ – Mithoron Dec 21 '17 at 17:12

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