2 Relative mass

DEFINITION: Relative Isotopic mass is the mass of one isotope compared to one twelfth of the mass of one atom of carbon-12 The relative atomic mass quoted on the periodic table is a weighted average of all the isotopes R.A.M =  (isotopic mass x % abundance) 100 For above example of Mg R.A.M = [(78.7 x 24) + (10.13 x 25) + (11.17 x 26)] /100 = 24.3 R.A.M =  (isotopic mass x relative abundance) total relative abundance If relative abundance is used instead of percentage abundance use this equation Calculating the Relative Atomic Mass of an Element Percentage Abundance 78.7 10.13 11.17 Relative Isotopic mass 24.00 25.00 26.00 Isotope Mg24 Mg25 Mg26 DEFINITION: Relative molecular mass is the average mass of a molecule compared to one twelfth of the mass of one atom of carbon-12 N Goalby chemrevise.org 2 Example: Calculate the relative atomic mass of Tellurium from the following abundance data: 124-Te relative abundance 2; 126-Te relative abundance 4; 128-Te relative abundance 7; 130-Te relative abundance 6 R.A.M = [(124×2) + (126×4) + (128×7) + (130×6)] 19 = 127.8 Example: Copper has two isotopes 63-Cu and 65-Cu. The relative atomic mass of copper is 63.5. Calculate the percentage abundances of these two isotopes. 63.55 = yx63 + (1-y)x65 63.55 = 63y +65 -65y 63.55 = 65 -2y 2y = 1.45 y = 0.725 %abundance 63-Cu =72.5% %abundance 65-Cu = 27.5% Cl has two isotopes Cl35 (75%) and Cl37(25%) Br has two isotopes Br79 (50%) and Br81(50%) These lead to the following spectra caused by the diatomic molecules 70 72 74 m/z relative abundance Cl35Cl35 +Cl35Cl37 +Cl37Cl37 + 158 160 162 m/z relative abundance Br79Br79 + Br79Br81 + Br81Br79 + Br81Br81 + Mass spectra for Cl2 and Br2 The 160 peak has double the abundance of the other two peaks because there is double the probability of 160 Br79 -Br81 + as can be Br79-Br81 and Br81-79. DEFINITION: Relative atomic mass is the average mass of one atom compared to one twelfth of the mass of one atom of carbon-12