Mass
In everyday usage, mass and "weight" are often used interchangeably. For instance, a person's weight may be stated as 75 kg. In a constant gravitational field, the weight of an object is proportional to its mass, and it is unproblematic to use the same unit for both concepts. But because of slight differences in the strength of the Earth's gravitational field at different places, the distinction becomes important for measurements with a precision better than a few percent, and for places far from the surface of the Earth, such as in space or on other planets. Conceptually, "mass" (measured in kilograms) refers to an intrinsic property of an object, whereas "weight" (measured in newtons) measures an object's resistance to deviating from its natural course of free fall, which can be influenced by the nearby gravitational field. No matter how strong the gravitational field, objects in free fall are weightless, though they still have 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