To answer this question, we must first determine the balanced chemical equation for the reaction between Mg and HCl. The reaction is as follows:

$$Mg(s) + 2HCl(aq) → MgCl_2(aq) + H_2(g)$$

Now, we need to determine the number of moles of Mg in 5.2 grams. To do this, we divide the mass of Mg by its molar mass. The molar mass of Mg is 24.31 g/mol.

$$5.2 \text{ g Mg} \times \frac{1 \text{ mol Mg}}{24.31 \text{ g Mg}} = 0.214 \text{ mol Mg}$$

According to the balanced chemical equation, 1 mole of Mg reacts with 2 moles of HCl. Therefore, we need 0.428 moles of HCl to completely react with 5.2 grams of Mg.

Finally, we need to determine the mass of HCl that corresponds to 0.428 moles. To do this, we multiply the number of moles by the molar mass of HCl. The molar mass of HCl is 36.46 g/mol.

$$0.428 \text{ mol HCl} \times 36.46 \text{ g HCl/mol} = 15.6 \text{ g HCl}$$

Therefore, the mass of HCl that would be required to completely react with 5.2 grams of Mg is 15.6 grams.