The limiting reagent is the reactant with the smallest number of moles divided by its coefficient, not the smallest mass: 16 g of methane burnt in 48 g of oxygen is limited by the oxygen, 0.75 against 0.997.
Methane is 0.997 mol and oxygen 1.5 mol, and the equation CH₄ + 2 O₂ → CO₂ + 2 H₂O wants twice as much oxygen as methane. Dividing by the coefficients, 0.997 ÷ 1 = 0.997 and 1.5 ÷ 2 = 0.75: the smaller, oxygen, runs out first, and it sets the product at 0.75 mol of carbon dioxide, 33.0 g.
It needs the reaction written as one balanced equation. With two reactions going at once, or a reactant that is also made in a side reaction, there is no single limiting reagent. A reactant that is a catalyst, or the solvent, does not appear in the equation and does not limit it.
Estimate with the rule, then check it against the calculator that models it properly.
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