Question 1

What does KOH absorption measure in eudiometry?

Accepted Answer

After water is removed, the collected dry combustion gas contains CO₂ along with excess O₂. KOH absorbs only CO₂, while excess O₂ remains in the receiver. At the same temperature and pressure, the decrease in total gas volume gives the CO₂ volume. CO₂ must not be collected directly over ordinary water because it is appreciably soluble and can dissolve in the water.

Question 2

Why does 22.7 L of carbon dioxide at STP correspond to 12 g of carbon?

Accepted Answer

NCERT uses 0 °C and 1 bar for STP, where one mole of an ideal gas occupies approximately 22.7 L. One mole of CO₂ contains one mole of carbon atoms, and one mole of carbon has a mass of 12 g.

Question 3

How is the percentage of hydrogen found from water?

Accepted Answer

The condensed water is weighed and its hydrogen fraction is calculated from the ratio 18 g H₂O to 2 g H. Dividing the recovered hydrogen mass by the original sample mass gives the percentage of hydrogen.

Question 4

What is the percentage composition of ethane?

Accepted Answer

Ethane has formula C₂H₆ and molar mass 30 g mol⁻¹. One mole contains 24 g of carbon and 6 g of hydrogen, giving 80% carbon and 20% hydrogen by mass.

Question 5

Can oxygen be found by difference for a pure hydrocarbon?

Accepted Answer

No. A pure hydrocarbon contains only carbon and hydrogen, so its percentages add to 100% and the oxygen percentage is zero. Oxygen by difference applies only to a measured compound that also contains oxygen.

Eudiometry: Burn a Hydrocarbon, Catch Its Carbon and Hydrogen

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