$1g$  of Mg is burnt in a vessel containing $0.5g$ of oxygen. The reactant remaining unreacted is

When 1g Mg is burned in a vessel containing 0.5g of oxygen (O₂), we need to determine which reactant remains unreacted. To solve this, we follow the steps below:

Step 1: Write the Balanced Chemical Equation

The reaction between magnesium and oxygen is represented as:

        2Mg + O₂ → 2MgO

From the balanced equation, 48g of Mg reacts with 32g of O₂ to form magnesium oxide (MgO).

Step 2: Determine the Amount of O₂ Required for 1g Mg

We know that 48g of Mg requires 32g of O₂. Using this ratio, we calculate:

Mass of O₂ required for 1g Mg:

        (32g O₂ / 48g Mg) × 1g Mg = 0.67g O₂

Thus, 1g Mg requires 0.67g of O₂.

Step 3: Compare Available and Required Oxygen

The available oxygen is 0.5g, which is less than the required 0.67g. This means oxygen (O₂) is the limiting reagent, and some magnesium (Mg) will remain unreacted.

Step 4: Calculate the Excess Mg

Using the same ratio, we determine the amount of magnesium required to completely react with 0.5g of O₂:

        (48g Mg / 32g O₂) × 0.5g O₂ = 0.75g Mg

So, only 0.75g of Mg is needed to react with 0.5g of O₂.

Step 5: Determine the Unreacted Mg

The initial amount of magnesium was 1g Mg. After the reaction:

        Excess Mg = Initial Mg - Reacted Mg
                  = 1g Mg - 0.75g Mg
                  = 0.25g Mg

Thus, 0.25g Mg remains unreacted.

Final Answer

The reactant remaining unreacted is 0.25g Mg, making the correct answer (A).