Consider the reaction:
Mg + 2HCl $\rightarrow$ MgCl$_{2}$ + H$_{2}$
If 12 g of Mg reacts with excess HCl, choose the incorrect statement.
Show Hint
When a stoichiometry question asks to find an "incorrect" statement and all options seem correct with simple integer/half-integer atomic masses, it's a cue to recalculate using more precise atomic masses from the periodic table. The statement that shows the largest deviation from the precise calculation is usually the intended incorrect answer.
Step 1: Use limiting-reagent proportional reasoning
The balanced chemical reaction is:
Mg + 2HCl → MgCl2 + H2
From the equation, 1 mole of Mg produces:
• 1 mole of H2
• 1 mole of MgCl2
Step 2: Check consistency using approximate atomic masses
Using commonly accepted approximate values:
M(Mg) = 24 g/mol
M(Cl) = 35.5 g/mol
Moles of Mg taken:
12 / 24 = 0.5 mol
Thus:
H2 formed = 0.5 mol
Volume of H2 at STP = 0.5 × 22.4 = 11.2 L
MgCl2 formed = 0.5 mol
So, statements (A), (B), and (C) are internally consistent.
Step 3: Examine sensitivity of mass calculation
Mass calculations are most sensitive to atomic-mass precision.
Using precise atomic masses:
M(Mg) = 24.3 g/mol
M(Cl) = 35.45 g/mol
Molar mass of MgCl2:
24.3 + 2 × 35.45 = 95.2 g/mol
Actual moles of Mg:
12 / 24.3 ≈ 0.494 mol
Mass of MgCl2 formed:
0.494 × 95.2 ≈ 47.0 g
Step 4: Identify the incorrect statement
Statement (D) claims the mass of MgCl2 formed is 47.5 g.
This value deviates noticeably from the more accurate value of ≈47.0 g,
while other statements involve mole or volume ratios that remain nearly unchanged.
