A 12.0 M acid solution that contains 75.0% acid by mass has a density of 1.57 g/mL. What is the identity of the acid? (A) HCl ( M = 36.5) (B) CH3CO2H (M = 60.0) (C)...

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Let the molecular weight of the acid be W.

Hence, 1000 ml 12.0 M acid solution contains 12W grams of pure acid.

Density of the solution = 1.57 g/ml

So, mass of 1000 ml of the solution = (1.57 * 1000) g = 1570 g

Again, mass of the acid in this solution = `75/100*1570` g = 1177.5 g

By condition:

12W = 1177.5 g

`rArr` W = `1177.5/12` g= 98.125 g

Therefore, the identity of the acid is option (D)  `H_3PO_4` (M = 98.0).

Sources:

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