Finding equivalent weight of acid given its reaction to form a salt


> 
> Find the equivalent weight of an acid if it is given that $\pu{1.321 g}$ acid reacts with magnesium to give $\pu{1.72 g}$ salt. Options:
> 
> 
> 1. $49$
> 2. $36.5$
> 3. $45$
> 4. none of above.
> 
> 
> 


The second choice is given as the answer. 


I tried to solve the question. I used the formula 


$$\frac{\text{wt. of acid}}{\text{equivalent weight of hydrogen} \text{ eq. wt. of anion say 'x'}} = \frac{\text{wt. of salt}}{\text{eq. wt. of magnesium eq. wt. of anion}}$$


And after solving I got $x=35.41$ taking equivalent weight of hydrogen as $1$, and $x=35.38$ taking equivalent weight of H as $1.008$. The answer is different in each case. Is this correct? But the correct answer given is $36.5$




The trick is to recognize that the acid and the salt share an anion $\ce{A}$, for example:


$$\ce{2HA + Mg -> MgA2 + H2}$$


Thus the equivalent weights of the acid and the salt are functions of the only one unknown, the equivalent weight of $\ce{A}$. 


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