Weight
In modern scientific usage, weight and mass are fundamentally different quantities: mass is an intrinsic property of matter, whereas weight is a "force" that results from the action of gravity on matter: it measures how strongly the force of gravity pulls on that matter. However, in most practical everyday situations the word "weight" is used when, strictly, "mass" is meant. For example, most people would say that an object "weighs one kilogram", even though the kilogram is a unit of mass.
Gas collecting tube
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 formula_1, the gas collecting tube is filled with the gas to be investigated and the overall mass formula_2 is measured. Then the aspirator sucks out much of the gas yielding a second overall mass formula_3 measurement. (The difference of masses represents the mass (formula_4) 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 formula_1. The gas collecting tube is weighed for a third and last time containing the liquid yielding the value formula_6. (The difference of masses of the nearly evacuated tube and the liquid-containing tube gives the mass (formula_7) of the sucked-in liquid, that took the place of the extracted amount of gas.) The given mass-density formula_8 of the liquid permits to calculate the displaced volume (formula_9). Thus giving mass formula_10 and volume formula_11 of the extracted amount of gas, consequently accessing its mass density (formula_12) under atmospheric pressure