Michael S. answered • 08/05/26
B.S. in Chemistry, Indiana University; Organic Chem Teaching Intern
5.04 g of aluminum
Every problem of this shape runs on the same three-step path: get to moles → use the coefficients → get back to the unit asked for. The balanced equation only works in the middle step, in moles — never in grams or litres.
2 Al(s) + 6 HCl(aq) → 2 AlCl3(aq) + 3 H2(g)
Step 1 — moles of HCl
Molarity is mol/L, so multiplying by volume in litres gives moles directly:
n(HCl) = (1.60 L)(0.350 mol/L) = 0.560 mol HCl
Step 2 — mole ratio from the equation
The coefficients say 2 Al react with 6 HCl, a 2:6 ratio that reduces to 1:3.
n(Al) = 0.560 mol HCl × (2 mol Al / 6 mol HCl) = 0.1867 mol Al
Sanity check on direction: aluminum should be the smaller number, since it takes three HCl to consume one Al. It is. If your Al value had come out larger than 0.560, you flipped the ratio.
Step 3 — convert to grams
m(Al) = (0.1867 mol)(26.98 g/mol) = 5.04 g
What "completely react with" is telling you
It means you want the exact stoichiometric amount — enough aluminum to consume all the acid with nothing left over on either side. So there is no limiting-reactant analysis to do here. That phrasing is the signal that the HCl amount is the one fixed quantity and the Al is what you are solving for.
Where this goes wrong
1. Inverting the ratio. Using 6/2 instead of 2/6 gives 1.68 mol and 45.3 g — nine times too much. Write the conversion as a fraction with the unit you are cancelling on the bottom and the ratio orients itself.
2. Applying the ratio to grams. Coefficients count particles, not mass. You must be in moles before the 2:6 means anything.
3. Volume in millilitres. Molarity is per litre. Here 1.60 L is already correct, but a problem giving 1600 mL needs the conversion first.
If a later part asks for the hydrogen produced, the same middle step handles it: 0.560 mol HCl × (3 H2 / 6 HCl) = 0.280 mol H2, which is 6.27 L at STP.
