Can someone please clarify that i've understood this correctly:
For the equation:
Mg(s) + 2HCl(aq) -> MgCl2(aq) + H2(g),
we know that n(Mg)=0.0551 and n(HCl)=0.0600.
Since the Mg is in excess, the amount reacting would follow the mole ratio. This means, 0.03mol of Mg would react with 0.0600 mol of HCl. This would produce 0.09mol of MgCl2 and H2 gas.
Looking at the mole ratio: you have 1:2:1:1
You are right at the point Mg is the excess reactant, hence the calculation will base on the mole of HCl. According to the mole ratio above, if you have 0.0600 mole of HCl, Mg will be halved that mole amount and equal 0.0300 mole. Other products also have the same mole ratio as Mg (1:1:1), therefore, MgCl2 and H2 will have the 0.0300 as well.
Remember that determining mole is based on the mole ratio on the equation you got. I think you are confused with the Conservation of Mass theory, which means the MASS of reactants must equal the mass of products. Note that this is mole, not mass.
Hope this helps!