When 1 dm³ of CO2 gas is passed over hot coke, the volume of gaseous mixture after complete reaction at STP becomes 1.4 dm³. The composition of the gaseous mixture at STP is:

**Step 1 – Write the balanced chemical equation**
When carbon (coke) is heated in the presence of carbon dioxide, the Boudouard reaction occurs:

$C_{}+{\mathrm{CO}}_2\to 2{\mathrm{CO}}_{}$

**Step 2 – Define the initial amount of CO₂**
At STP, 1 dm³ of any ideal gas corresponds to 1 mol. Thus the initial moles of CO₂ = 1 mol.

**Step 3 – Let *x* be the moles of CO₂ that react**
If *x* mol of CO₂ react, the equation shows that 2*x* mol of CO are produced.

**Step 4 – Express the remaining amounts after reaction**
Remaining CO₂ = 1 mol – *x*
Produced CO = 2 *x* mol

**Step 5 – Use the final total volume (moles) of the gas mixture**
The problem states that after the reaction the total volume is 1.4 dm³, i.e., 1.4 mol of gas.

**Step 6 – Set up the total‑mole equation**
Total moles = (remaining CO₂) + (produced CO) = (1 – *x*) + 2*x* = 1 + *x*

**Step 7 – Solve for *x***
\[ 1 + x = 1.4 \;\Rightarrow\; x = 0.4\;\text{mol} \]

**Step 8 – Calculate the individual gas volumes**
CO produced = 2 × 0.4 mol = 0.8 mol ⇒ 0.8 dm³ of CO
CO₂ remaining = 1 – 0.4 mol = 0.6 mol ⇒ 0.6 dm³ of CO₂

**Result**
The gaseous mixture at STP consists of **0.8 dm³ of CO** and **0.6 dm³ of CO₂**.