Magnesium burns: 2Mg + O₂ → 2MgO. A jar holds 6.0 g of oxygen, and 6.0 g of magnesium ribbon burns…

Step 1. Moles. Magnesium $\dfrac{6.0}{24.31} = 0.2468$ mol; oxygen $\dfrac{6.0}{32.00} = 0.1875$ mol.

Step 2. Rounds. Magnesium $0.1234$, oxygen $0.1875$. Magnesium is limiting, and the extent is $0.1234$.

Step 3. Product. $2 \times 0.1234 = 0.2468$ mol of MgO, or $0.2468 \times 40.31 = 9.95$ g.

Step 4. Oxygen left. $0.1875 - 0.1234 = 0.0641$ mol, or $2.05$ g.