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. 23.5 K cals
B. 2.35 K cals
C. 94.0 K cals
D. 31.3 K cals

Explanation

To solve the problem of how much heat is liberated when 3 grams of graphite is burnt in excess oxygen, we can follow these steps:

### Step 1: Understand the Reaction
The combustion of graphite (carbon) in oxygen can be represented by the following chemical equation:
\[ C + O_2 \rightarrow CO_2 \]
The heat of formation of carbon dioxide (CO2) is given as -94.0 kilocalories. This means that when 1 mole of carbon is burned, 94.0 kilocalories of heat is released.

### Step 2: Determine the Molar Mass of Carbon
The molar mass of carbon (C) is approximately 12 grams per mole. This means that 1 mole of carbon weighs 12 grams.

### Step 3: Calculate the Heat Released per Gram of Carbon
Since 1 mole of carbon (12 grams) releases 94.0 kilocalories, we can calculate the heat released per gram of carbon:
\[ \text{Heat released per gram} = \frac{94.0 \text{ kcal}}{12 \text{ g}} \approx 7.83 \text{ kcal/g} \]

### Step 4: Calculate the Heat Released for 3 Grams of Carbon
Now, we can find out how much heat is released when 3 grams of carbon is burnt:
\[ \text{Heat released} = 3 \text{ g} \times 7.83 \text{ kcal/g} \approx 23.49 \text{ kcal} \]

### Step 5: Round the Answer
Rounding this value to one decimal place gives us approximately 23.5 kilocalories.

### Final Answer
The quantity of heat liberated when 3 grams of graphite is burnt in excess oxygen is approximately **23.5 kilocalories**.

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