Answer: (i) 0.261 moles of HCl, (ii) 0.131 moles of
and (iii) 12.5 grams of 
Explanation: The balanced equation for the formation of HCl by the reaction magnesium chloride with water is:

From above balanced equation, there is 1:2 mol ratio between magnesium chloride and HCl.
As per the given information, 5.85 L of HCl gas is produced. here, temperature and pressure is not mentioned, so let's assume STP conditions.
At STP, volume of 1 mol of a gas is 22.4 L. With the help of this, we can calculate the moles of HCl present in 5.85 L.

= 0.261 mol HCl
As there is 1:2 mol ratio between magnesium chloride and HCl, we can calculate the moles of magnesium chloride from the above calculated moles of HCl as:

= 0.131 mol 
Molar mass of magnesium chloride is given as 95.2 gram per mol. On multiplying the moles by molar mass we can calculate its mass reacted.

= 12.5 g 