Gas collecting tube

• Gas collecting tubes, made of glass, with 2 stopcocks with glass plugs, without septum
• as above, with septum
• with PTFE plug, without septum
• as above, with septum

Capacity of gas collecting tubes 150, 250, 350, 500 and 1.000 ml

The mass density of a gas can be measured by using a gas collecting tube, an analytical balance and an aspirator. The mass and volume of a displaced amount of gas are determined: At atmospheric pressure ${\displaystyle p}$, the gas collecting tube is filled with the gas to be investigated and the overall mass ${\displaystyle m_{full}}$ is measured. Then the aspirator sucks out much of the gas yielding a second overall mass ${\displaystyle m_{sucked}}$ measurement. (The difference of masses represents the mass (${\displaystyle m=m_{full}-m_{sucked}}$) of the extracted amount of gas.) Finally, the nearly evacuated gas collecting tube is allowed to suck in an outgassed liquid, usually previously heated water, which is offered under atmospheric pressure ${\displaystyle p}$. The gas collecting tube is weighed for a third and last time containing the liquid yielding the value ${\displaystyle m_{containingliquid}}$. (The difference of masses of the nearly evacuated tube and the liquid-containing tube gives the mass (${\displaystyle m_{liquid}=m_{containingliquid}-m_{sucked}}$) of the sucked-in liquid, that took the place of the extracted amount of gas.) The given mass-density ${\displaystyle \rho _{liquid}}$ of the liquid permits to calculate the displaced volume (${\displaystyle V=m_{liquid}/\rho _{liquid}}$). Thus giving mass ${\displaystyle m}$ and volume ${\displaystyle V}$ of the extracted amount of gas, consequently accessing its mass density (${\displaystyle \rho ={\frac {m}{V}}}$) under atmospheric pressure.[1]

The mass density of a fluid can be measured by using a gas collecting tube, an analytical balance and two other fluids of known mass densities, preferably a gas and a liquid (with mass densities ${\displaystyle \rho _{gas}}$, ${\displaystyle \rho _{liquid}}$). Overview: First, mass measurements get the volume ${\displaystyle V}$ and the evacuated mass ${\displaystyle m_{evactube}}$ of the gas collecting tube; secondly, these two are used to measure and calculate the mass-density ${\displaystyle \rho }$ of the investigated fluid.

Fill the gas collecting tube with one of those fluids of given mass density and measure the overall mass, do the same with the second one giving the two mass values ${\displaystyle m_{fullgas}}$, ${\displaystyle m_{fullliquid}}$. Consequently, for those two fluids, the definition of mass density can be rewritten:

${\displaystyle \rho _{gas}={\frac {m_{fullgas}-m_{evactube}}{V}}\,}$

${\displaystyle \rho _{liquid}={\frac {m_{fullliquid}-m_{evactube}}{V}}}$

These two equations with two unknowns ${\displaystyle m_{evactube}}$ and ${\displaystyle V}$ can be solved by using elementary algebra:

${\displaystyle V={\frac {m_{fullliquid}-m_{fullgas}}{\rho _{liquid}-\rho _{gas}}}\,}$

${\displaystyle m_{evactube}={\frac {\rho _{gas}\cdot m_{fullliquid}-\rho _{liquid}\cdot m_{fullgas}}{\rho _{liquid}-\rho _{gas}}}}$

(The relative error of the result significantly depends on the relative proportions of the given mass densities and the measured masses.)

Now fill the gas collecting tube with the fluid to be investigated. Measure the overall mass ${\displaystyle m_{full}}$ to calculate the mass of the fluid inside the tube ${\displaystyle m=m_{full}-m_{evactube}}$ yielding the desired mass density ${\displaystyle \rho ={\frac {m}{V}}}$.

If the gas is a pure gaseous chemical substance (and not a mixture), with the mass density ${\displaystyle \rho ={\frac {m}{V}}}$, then using the ideal gas law permits to calculate the molar mass ${\displaystyle M}$ of the gaseous chemical substance:

${\displaystyle M={\frac {m\cdot R\cdot T}{p\cdot V}}\,}$

Where ${\displaystyle R}$ represents the universal gas constant, ${\displaystyle T}$ the absolute temperature at which the measurements took place.

• Source of the notion "gas collecting tube", among others