Question

A 4.00g piece of magnesium is added to 200mL of 1.40 mol/l HC.

Identify the limiting reactant, and calculate the mass of hydrogen gas that should be produced

Answer 1

This is a limiting reagent (LR) problem. You know that because amounts are given for BOTH reagents. I suppose you means HCl for HC.
1. Write and balance the equation.
2. Convert grams Mg to mols. mols = grams/atomic mass = ?
3. Convert HCl to mols. mols HCl = M x L = ?
4. Using the coefficients in the balanced equation, convert mols Mg to mols H2.
5. Do the same and convert mols HCL to mols H2.
6. It is likely that the values for mols H2 produced in the above steps will not be the same which means one of them is not right. The correct value in LR problems is ALWAYS the smaller value and the reagent responsible for that value is the LR.
7. Using the smaller value, convert mols H2 to grams H2. g = mols x molar mass = ? This is the theoretical yield.