In combustion method for analysing molecular formula of gaseous hydrocarbon, is pressure of the gaseous mixture always constant?

Question

I understand the we have to look at the volumes, but the fact that volume of a gas is directly proportional to the number of moles of the gas holds only at constant pressure. Also we are applying Gay-Lussacs law, so shouldnt the pressure be constant at all times?

I don't understand why the gaseous mixture is taken to be at constant pressure, the diagram of the eudiometer tube is not able to convince me that the pressure of the mixture is constant.

If the volume of the gas is changing, the height of the water column inside the tube is also changing. Since atmospheric pressure is constant then on changing height of water column shouldn't the pressure of the gaseous mixture also change?

Please explain how the pressure of the mixture is constant.

Answer 1

Mathematically constant is not the same as constant in context of measurement.

For the latter,

• below the error threshold it is considered constant
• above the threshold are applied corrections.

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The volume measurement can be adjusted in such a way outer and inner water levels are aligned anth therefore volumes are measured at constant pressure(safe the trend of atmospheric pressure).

Or, corrections can be made for hydrostatic pressure.

Or, deviations can be neglected, depending on scenario demand on result accuracy.

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The topic has in fact nothing to do specifically with chemistry. It is the general topic of mathematical versus scientific precision.

A measured quantity is within a given scenario considered constant if:

• Deviations are below measurement resolution threshold
• Deviations are observed but negligible compared to other error sources.
• Deviations are not negligible wrt other error sources, but can be neglected in context of required accuracy of the result vulue.

Answer 2

Is pressure in the eudiometer tube always constant? Probably not if combustion is involved.

However, once allowed to cool, pressures will be constant. Volumes change. This is done so volume (as residual oxygen) can be measured and compared with the original volume of oxygen.

Hydrocarbon analysis is based on ratio of carbon (atomic weight 12) and hydrogen (atomic weight 1).

More moles of oxygen are consumed for a given mass of hydrocarbon if the ratio of hydrogen content is higher, as in CH4 (4:1) as compared with C8H18 (2.25:1).

Alkali is used to strip CO2 from the combustion mixture, leaving only residual oxygen.

Atmospheric pressure is used to keep pressure in the reactor constant (just hope the barometer does not change much while running the experiment). Volume changes based on moles Oxygen consumed by combustion.