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Carbon dioxide formula molar mass8/31/2023 Thus if Gas A is the " i-th" substance as in the expression immediately above, the summation runs from i=1 through i=2. Volume fractions are often called partial volumes:ĭon't let this type of notation put you off! The summation sign Σ (Greek Sigma) simply means to add up the v's (volumes) of every gas. We can specify the composition of this mixture by saying that the volume fraction of A is 1/3 and that of B is 2/3. So if a partitioned container has two volumes of gas A in one section and one mole of gas B in the other (both at the same temperature and pressure), and we remove the partition, the volume remains unchanged. This means that the volumes of gases, unlike those of solids and liquids, are additive. You will be expected to carry out calculations of this kind, converting between molecular weight and gas density.įrom Avogadro's Law we know that "equal volumes contains equal numbers of molecules". Pay careful attention to the examples of gas density calculations shown here and in your textbook. The weight of the gas divided by its STP volume yields the density of the gas, and the density multiplied by 22.4 mol –1 gives the molecular weight. One simply measures the weight of a known volume of gas and converts this volume to its STP equivalent, using Boyle's and Charles' laws. Determination of the molecular weight of a gas from its density is known as the Dumas method, after the French chemist Jean Dumas (1800-1840) who developed it. Gas densities are now measured in industry by electro-mechanical devices such as vibrating reeds which can provide continuous, on-line records at specific locations, as within pipelines. This is one of many important examples of how a macroscopic measurement (one made on bulk matter) can yield microscopic information (that is, about molecular-scale objects.) More importantly, if we can measure the density of an unknown gas, we have a convenient means of estimating its molecular weight. Note: there is no need to look up a "formula" for this calculation simply combine the molar mass and molar volume in such a way as to make the units come out correctly.
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