Question

A 105.39 mg sample of unknown was submitted to combustion analysis. The sample yielded 330.60 mg of carbon dioxide and 135.37 mg of water. What is the percentage composition of carbon in the unknown sample?

Answer 1

To determine the percentage composition of carbon in an unknown sample from a combustion analysis, you calculate the moles of carbon dioxide produced, convert that to moles of carbon, and subsequently the mass of carbon. Then, divide this mass of carbon by the total mass of the sample and multiply by 100 to get the percentage composition, which is 85.61% in this case.

Step-by-step explanation:

To calculate the percentage composition of carbon in the unknown sample, you need to determine how much carbon is present in the carbon dioxide produced by the combustion. Every one mole of carbon dioxide contains one mole of carbon, since the molecular formula of carbon dioxide is CO2. The molar mass of CO2 is 44.01 g/mol, and the molar mass of carbon is 12.01 g/mol. By dividing the mass of the CO2 produced (330.60 mg) by its molar mass, you get the moles of CO2, and therefore moles of carbon. Then by multiplying the moles of carbon by its molar mass, you can find the mass of carbon.

Here's the calculation step-by-step:

1. Convert 330.60 mg of CO2 to grams: 330.60 mg ÷ 1000 = 0.33060 g.
2. Calculate moles of CO2: 0.33060 g ÷ 44.01 g/mol = 0.007514 moles of CO2.
3. Calculate moles of C in CO2: 0.007514 moles CO2 × 1 mole C/mole CO2 = 0.007514 moles C.
4. Calculate the mass of carbon: 0.007514 moles C × 12.01 g/mol = 0.090216 g of C.
5. Find the percent composition of C: (0.090216 g C ÷ 0.10539 g sample) × 100 = 85.61% C.

The percentage composition of carbon in the unknown sample is therefore 85.61%.