R For Helium
About Helium; 1 cubic centimeter of Helium weighs 0.0001785 gram g 1 cubic inch of Helium weighs 0.00010318 ounce oz Helium weighs 0.0001785 gram per cubic centimeter or 0.1785 kilogram per cubic meter, i.e. Density of helium is equal to 0.1785 kg/m³; at 0°C (32°F or 273.15K) at standard atmospheric pressure.In Imperial or US customary measurement system, the density is equal to. At left is a helium spectral tube excited by means of a 5000 volt transformer. At the right of the image are the spectral lines through a 600 line/mm diffraction grating. Standard Atmosphere, 1976 (USSA1976) defines the gas constant R ∗ as: R ∗ = 8.314 32 × 10 3 N⋅m⋅kmol −1 ⋅K −1. Note the use of kilomole units resulting in the factor of 1,000 in the constant. The USSA1976 acknowledges that this value is not consistent with the cited values for the Avogadro constant and the Boltzmann constant.
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The speed of sound in an ideal gas is given by the relationship

 R = the universal gas constant = 8.314 J/mol K,
 T = the absolute temperature
 M = the molecular weight of the gas in kg/mol
 γ = the adiabatic constant, characteristic of the specific gas
For air, the adiabatic constant γ = 1.4 and the average molecular mass for dry air is 28.95 gm/mol. This leads to
Doing this calculation for air at 0°C gives v_{sound} = 331.39 m/s and at 1°C gives v_{sound} = 332.00 m/s. This leads to a commonly used approximate formula for the sound speed in air:
For temperatures near room temperature, the speed of sound in air can be calculated from this convenient approximate relationship, but the more general relationship is needed for calculations in helium or other gases.
Three Uses Of Helium
The calculation above was done for dry air, and moisture content in the air would be expected to increase the speed of sound slightly because the molecular weight of water vapor is 18 compared to 28.95 for dry air. A revised average molecular weight could be calculated based on the vapor pressure of water in the air. However, the assumption of an adiabatic constant of γ = 1.4 used in the calculation is based upon the diatomic molecules N_{2} and O_{2} and does not apply to water molecules. So the detailed modeling of the effect of water vapor on the speed of sound would have to settle on an appropriate value of γ to use.
R For Helium
Table of sound speeds 