Question

If 2.00 g of mg reacts completely with 50.0 ml of hcl solution, what is the molarity of the hcl solution? mg(s) + 2hcl(aq) → mgcl2(aq) + h2(g)

Answer 1

The molarity of HCl is calculated as below

find the moles  of   mg used
moles = mass /molar mass

= 2.00g/24 g/mol=0.083 moles
 write the  reacting equation
Mg(s) +2HCl (aq)= MgCl2(aq) + H2(g)

by use of mole ratio between Mg to HCl  which is 1  :2 the moles of HCl is therefore
= 0.083 x2 =0.166 moles

molarity of HCl = moles of HCl/ volume in dm^3

that is 0.166/50 x1000= 3.3 M of HCl

Answer 2

The balanced equation for the reaction between Mg and HCl is as follows;
Mg + 2HCl ---> MgCl₂ + H₂
stoichiometry of Mg to HCl is 1:2
number of moles of Mg reacted - 2.00 x 10⁻³ g / 24 g/mol = 0.0833 x 10⁻³ mol 
according to molar ratio 
number of HCl moles reacted - 0.0833 x 10⁻³ x 2 mol = 0.167 x 10⁻³ mol 
number of HCl moles in 50.0 mL -  0.167 x 10⁻³ mol 
therefore number of HCl moles in 1000 mL -  0.167 x 10⁻³ mol / 0.0500 L 
molarity of HCl - 0.00334 M