When 0.01 mol of an organic compound containing 60% carbon was burnt completely, 4.4 g of CO₂ was produced. The molar mass of compound is _____________ g mol⁻¹ (Nearest integer).

Solution

Chemistry ma’am · 3 steps

Follow the carbon: every C atom in the compound ends up in CO₂, so count the carbon first.

1. Moles of carbon. 4.4 g of CO₂ is 4.4/44 = 0.1 mol. Each CO₂ has one C atom, so the sample contained 0.1 mol of C, which is 0.1 × 12 = 1.2 g of carbon.
2. Mass of sample. Carbon is 60% of the compound by mass. So the sample that was burnt weighed 1.2/0.60 = 2 g.

   M = mass/moles = (2 g)/(0.01 mol) = 200 g mol⁻¹

3. So. The molar mass of the compound is 200 g mol⁻¹.

The common slip: 2. Stopping at the sample mass gives 2, not 200. That 2 g belongs to only 0.01 mol, so you still divide by 0.01.

The idea to keep. In combustion questions, carbon is conserved: moles of CO₂ give moles of C. Use the percentage to get the sample mass, then divide by moles of sample for the molar mass.

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