# Gas collecting tube

The characterization gas collecting tube describes an oblong gas-tight container with one valve at either end. Usually such a container has a gauged volume, has a cylindrical shape and is made of glass. Gas collecting tubes are used for science-related purposes; for taking samples of gases.

## Versions

* 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 can be 150, 250, 350, 500, or 1000 ml.

## Measurement of the mass density of a gas – aspirator method

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 $p$, the gas collecting tube is filled with the gas to be investigated and the overall mass $m_{full}$ is measured. Then the aspirator sucks out much of the gas yielding a second overall mass $m_{sucked}$ measurement. (The difference of masses represents the mass ($m=m_{full}-m_{sucked}$) 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 $p$. The gas collecting tube is weighed for a third and last time containing the liquid yielding the value $m_{containingliquid}$. (The difference of masses of the nearly evacuated tube and the liquid-containing tube gives the mass ($m_{liquid}=m_{containingliquid}-m_{sucked}$) of the sucked-in liquid, that took the place of the extracted amount of gas.) The given mass-density $\rho _{liquid}$ of the liquid permits to calculate the displaced volume ($V=m_{liquid}/\rho _{liquid}$). Thus giving mass $m$ and volume $V$ of the extracted amount of gas, consequently accessing its mass density ($\rho ={\frac {m}{V}}$) under atmospheric pressure.

## Measurement of the mass density of a gas – two fluid method

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 $\rho _{gas}$, $\rho _{liquid}$). Overview: First, mass measurements get the volume $V$ and the evacuated mass $m_{evactube}$ of the gas collecting tube; secondly, these two are used to measure and calculate the mass-density $\rho$ 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 $m_{fullgas}$, $m_{fullliquid}$. Consequently, for those two fluids, the definition of mass density can be rewritten:
    $\rho _{gas}={\frac {m_{fullgas}-m_{evactube}}{V}}\,$
    $\rho _{liquid}={\frac {m_{fullliquid}-m_{evactube}}{V}}$
These two equations with two unknowns $m_{evactube}$ and $V$ can be solved by using elementary algebra:
    $V={\frac {m_{fullliquid}-m_{fullgas}}{\rho _{liquid}-\rho _{gas}}}\,$
    $m_{evactube}={\frac {\rho _{gas}\cdot m_{fullliquid}-\rho _{liquid}\cdot m_{fullgas}}{\rho _{liquid}-\rho _{gas}}}$
(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 $m_{full}$ to calculate the mass of the fluid inside the tube $m=m_{full}-m_{evactube}$ yielding the desired mass density $\rho ={\frac {m}{V}}$.

## Molar Mass from the Mass Density of a Gas

If the gas is a pure gaseous chemical substance (and not a mixture), with the mass density $\rho ={\frac {m}{V}}$, then using the ideal gas law permits to calculate the molar mass $M$ of the gaseous chemical substance:
    $M={\frac {m\cdot R\cdot T}{p\cdot V}}\,$
Where $R$ represents the universal gas constant, $T$ the absolute temperature at which the measurements took place.