How do you know if a problem is a limiting reactant problem?

Failing that you must write the stoichiometrically balanced equation..and then examine the stoichiometric equivalence of each reagent... And combustion reactions generally offer much opportunity for this sort of question .... consider methane.

#CH_4(g)+2O_2(g)rarrCO_2(g) + 2H_2O(l) + Delta#

The equation is stoichiometrically balanced...garbage out equals garbage in. In general dioxygen will be present in excess, but incomplete combustion COULD occur (and does in higher hydrocarbons to give #CO# and #C# as products of incomplete oxidation...)

For say diesel, #C_12H_24(l) +33/2 O_2(g) rarr 10CO_2(g) + CO(g) + C(s) + 12H_2O(l)#

Dioxygen gas is conceived to be the limiting reagent in that the products of incomplete combustion are observed in the reaction mix. Of course in BOTH internal combustion, and diesel engines, incomplete combustion is known to occur...

SO in answer to your question you must have (i) a stoichiometrically balanced equation, and (ii) equivalent quantities of reagents in order to make the assessment...

A limiting reactant question will give you the masses or moles of the reactants in a given reaction, and ask how much of a certain product can be produced.

You will need to use stoichiometry to determine how much of the product can be produced by each reactant. The reactant that produces the least amount of the product is the limiting reactant, and it determines the maximum amount of the product that can be produced.

Example 1

Consider the following reaction:

#"2Al + 6HBr"##rarr##"2AlBr"_3 + "3H"_2"#

When #"3.22 moles Al"# reacts with #"4.96 moles HBr"#, how many moles of #"H"_2"# can be produced? What is the limiting reactant?

Example 2

Consider the following reaction:

#"4Al"_2"O"_3 + "9Fe"##rarr##"3Fe"_3"O"_4 + "8Al"#

When #"25.4 g Al"_2"O"_3"# reacts with #"10.2 g Fe"#, how many grams of #"Al"# can be produced? What is the limiting reactant?