If the standard molar enthalpy change for combustion of graphite powder is -2.48 × 10² kJ mol⁻¹, the amount of heat generated on combustion of 1 g of graphite powder is ___________ kJ. (Nearest integer)

Solution

Chemistry ma’am · 3 steps

The enthalpy is given per mole, so first turn 1 g of graphite into moles and then scale the heat.

1. Moles of graphite. Graphite is pure carbon, with molar mass 12 g mol⁻¹. So 1 g of graphite is 1/12 mol.
2. Heat per mole. Burning 1 mol of graphite gives Δ_c H° = -2.48 × 10² = -248 kJ mol⁻¹. The minus sign means heat is released, so 248 kJ is given out per mole.

   q = 1/12 mol × 248 kJ mol⁻¹ = 20.67 kJ

3. So. Burning 1 g of graphite generates about 20.67 kJ of heat, which rounds to 21.

The common slip: 248. Using the molar value directly, without converting 1 g to 1/12 mol, gives 248, not 21.

The idea to keep. Molar enthalpy is heat per mole. For a given mass, find the moles first, then multiply by the magnitude of Δ H; the sign only tells you heat is released.

Answer from the NTA final answer key. Solution by PrepWiser.