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| RÉDACTEURS: |
 AnglaisAditya Vardhan Eni Generalic |
 FrançaisMichel Ditria |
 CroateEni Generalic Marija Bralic Slobodan Brinic |
 AllemandMarc Hens |
 Italienrédacteurs recherchés |
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R |
Radioactivity | Raoult's law | Rare earth elements | Reference electrode | Relative atomic mass | Relative molecular mass |
- Radioactivity
- Radioactivity is the spontaneous emission of radiation and atomic particles from unstable elements. Radioactivity is measured in terms of half-lives and is not affected by physical state or chemical combination.
- Raoult's law
- Raoult's law is the expression for the vapor pressure pi of component i in an ideal solution
pi = pio xi where xi is the mole fraction of component i and pio the vapor pressure of the pure substance i.
- Rare earth elements
- Rare earth elements are the elements Sc, Y, and the lanthanides (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu).
- Reference electrode
- Reference electrode is an electrode whose potential is known and against which potentials of other electrodes can be measured. The potential of a reference electrode is completly independent of the analyte concentration.
Table: Potential of a reference electrodes
Potential vs. SHE, V Calomel electrode Ag / AgCl T, °C 0.1 M 3.5 M Saturate 3.5 M Saturate 15 0.3362 0.254 0.2511 0.212 0.209 20 0.3359 0.252 0.2479 0.208 0.204 25 0.3356 0.250 0.2444 0.205 0.199 30 0.3351 0.248 0.2411 0.201 0.194 35 0.3344 0.246 0.2376 0.197 0.189 - Relative atomic mass
- The ratio of the average mass per atom of an element to 1/12 of the mass of nuclide 12C. An atomic weight can be defined for a sample of any given isotopic composition. The standard atomic weight refers to a sample of normal terrestrial isotopic composition. The term relative atomic mass is synonymous with atomic weight.
- Relative molecular mass
- Relative molecular weight of the substance is defined as the sum of relative atomic weights of the elements, multiplied by the number of atoms in the formula of this substance. For example, when calculating the relative molecular weight (Mr) of sulphuric acid (H2SO4)
Mr(H2SO4) = 2×Ar(H) + Ar(S) + 4×Ar(O) = 2×1.0079 + 32.066 + 4×15.999 = 2.0158 + 32.066 + 63.996 = 98,078
