The equation of our reaction is:
Mg (s) + 2 HCl (aq) ----> MgCl₂ (aq) + H₂ (g)
According to the coefficients of the reaction 1 mol of Mg will react with 2 moles of HCl to produce 1 mol of MgCl₂ and 1 mol of H₂.
So 1 mol of H₂ is produced by 1 mol of Mg. We can use that relationship to find the number of moles of Mg (metal).
1 mol of H₂ = 1 mol of Mg
moles of Mg = 0.00165 moles of H₂ * 1 mol of Mg/(1 mol of H₂)
moles of Mg = 0.00165 moles
Answer: 0.00165 moles of metal
mass of the metal = 0.039 g
moles of metal = 0.00165 moles
molar mass of metal = 0.039 g/0.00165 moles
molar mass of metal = 23.63 g/mol
Answer: 23.63 g/mol is the molar mass of the metal