Question

Calculate change in enthalpy when 39 g acetylene is completely burnt with oxygen and enthalpy of combustion of acetylene is $-1300\ \text{kJ/mol}$. (At. mass C = 12, H = 1)

• A. $-975\ \text{kJ}$
• B. $-650\ \text{kJ}$
• C. $-1950\ \text{kJ}$
• D. $-1600\ \text{kJ}$

Hint:

1 mole of acetylene weighs 26 g. The problem asks for 39 g, which is exactly $1.5$ times the weight of 1 mole. Therefore, simply scale the energy by 1.5: $-1300 \times 1.5 = -1950\ \text{kJ}$!

Answer 1

The Correct Option is C

Step 1: Find the molar mass.
Acetylene $C_2H_2$ has mass $2(12) + 2(1) = 26$ g per mole.

Step 2: Find the moles.
$n = \dfrac{39}{26} = 1.5$ mol.

Step 3: Scale the heat.
Each mole gives off 1300 kJ, so $\Delta H = 1.5 \times (-1300) = -1950$ kJ.
\[ \boxed{-1950\ \text{kJ},\ \text{option 3}} \]