Heat of formation of `CO_(2)` is -94.0 K. cal. What would be the quantity of heat liberated, when 3 g of graphite is burnt in excess of oxygen :-

Options

A. 23.5Kcal
B. `2.35 " Kcal"`
C. `94.0 " Kcal"`
D. `31.3" Kcal"`

Answer

To solve the problem of calculating the quantity of heat liberated when 3 g of graphite is burnt in excess of oxygen, we can follow these steps:

### Step 1: Understand the Reaction
The combustion of graphite (carbon) in the presence of oxygen produces carbon dioxide (CO₂). The heat of formation of CO₂ is given as -94.0 kcal, which indicates that this amount of heat is released when one mole of CO₂ is formed from its elements (graphite and oxygen).

### Step 2: Determine Molar Mass of Graphite
The molar mass of carbon (graphite) is approximately 12 g/mol.

### Step 3: Calculate Moles of Graphite
To find the number of moles of graphite in 3 g, we use the formula:
\[ \text{Number of moles} = \frac{\text{mass}}{\text{molar mass}} \]
Substituting the values:
\[ \text{Number of moles of graphite} = \frac{3 \text{ g}}{12 \text{ g/mol}} = 0.25 \text{ mol} \]

### Step 4: Relate Moles of Graphite to Heat Released
Since the heat of formation of CO₂ is -94.0 kcal for 1 mole of graphite, we can calculate the heat released for 0.25 moles of graphite:
\[ \text{Heat released} = \text{Number of moles} \times \text{Heat of formation} \]
Substituting the values:
\[ \text{Heat released} = 0.25 \text{ mol} \times (-94.0 \text{ kcal/mol}) = -23.5 \text{ kcal} \]

### Step 5: Conclusion
The quantity of heat liberated when 3 g of graphite is burnt in excess of oxygen is 23.5 kcal (the negative sign indicates that it is an exothermic reaction).

### Final Answer
**The quantity of heat liberated is 23.5 kcal.**

---