[gt5hz]

The question is this.

You have a piece of magneisium of unknown mass and a beaker of water in which is dissolved an unknown amount of hydrogen chloride. Design an experiment to determine which reactant is the limited reagent.

The BALANCED chemical equation illustrating the reaction is as follows:

Mg (s) + 2HCl (aq) --> MgCl2 (aq) + H2 (g)

My approach: The way I'm interpreting this problem is that the products are irrelevent. The main focus is on the reactants and to determine the limited reagent.

I'd plan to get an electronic balance, mass the magnesium, and then find the molar mass of Magnesium using a table of elements. After which, I'd do the same exact thing for the hydrochloric acid (mass an empty beaker of the same type, mass the beaker with the HCl, find the mass of the HCL alone, and then find molar mass as depicted in the equation ---> 2 [1.0 + 35.5]). Then, I'd use stoichiometric calculations to determine the reactant with more moles, which would be the limited reageant.

Would this be correct? I feel like I'm missing something.

[nj_bartel]

Couldn't you just dump the Mg into the HCl, and

a) if there's metal leftover, Mg was in excess
b) if there's no metal leftover, take the pH - very low pH indicates excess HCl, essentially neutral pH indicates stoichiometrically equal

Why would the exact mass, or even the chemical equation be necessary?

[macman104]

They point out specifically that you do not know the mass.  You have to assume that you cannot determine this type of thing.  Follow nj.

[gt5hz]

Couldn't you just dump the Mg into the HCl, and

a) if there's metal leftover, Mg was in excess
b) if there's no metal leftover, take the pH - very low pH indicates excess HCl, essentially neutral pH indicates stoichiometrically equal

Why would the exact mass, or even the chemical equation be necessary?

Currently my chemistry class is on the second unit; Quantities in Chemical Reactions. We haven't even learned anything in B, this unit is completely math. The question is divided in subquestions, where the chemical equation is explicably provided in part a. With something like this, I assume it'd have to be done mathmetically, through stoichiometry calculations.

[ARGOS++]

Dear gt5hz;

There is absolutely no mathematical or stoichiometrical way to determine the limiting reactant with only unknown masses! That’s why they ask for an experiment to determine it, and an experiment is all times obliged with an observation/measurement as its result!

So nj_batel’s experiment is the "only" way to estimate the limiting reactant!

Good Luck!
                    ARGOS⁺⁺