Question

Complete combustion of 3 g of ethane gives x × 10$^{22}$ molecules of water. The value of x is _________. (Round off to the Nearest Integer). [Use : $N_A$ = 6.023 × 10$^{23}$; Atomic masses in u : C : 12.0 ; O : 16.0 ; H : 1.0]

Hint:

Always balance the hydrocarbon combustion equation first using the general formula: $C_xH_y + (x + \frac{y}{4})O_2 \rightarrow xCO_2 + \frac{y}{2}H_2O$.

Answer 1

Correct Answer: 18

Step 1: Balanced combustion equation for ethane ($C_2H_6$): $C_2H_6 + \frac{7}{2}O_2 \rightarrow 2CO_2 + 3H_2O$.
Step 2: Molar mass of $C_2H_6 = (2 \times 12) + (6 \times 1) = 30$ g/mol.
Step 3: Moles of ethane $= \frac{3 \text{ g}}{30 \text{ g/mol}} = 0.1$ mol.
Step 4: Moles of $H_2O$ produced $= 3 \times 0.1 = 0.3$ mol.
Step 5: Number of molecules $= 0.3 \times 6.023 \times 10^{23} = 1.8069 \times 10^{23} = 18.069 \times 10^{22}$. The value of $x \approx 18$.