Mass spectroscopy and Ar calculations                                 

1)   A sample of sulphur has three isotopes: sulphur-32, sulphur-33 and sulphur-34. Calculate the relative atomic mass from the data below:  Give your answer to the appropriate number of decimals places.

 

percentage abundance

95.02

0.76

4.22

Isotopic mass

32

33

34


RAM = S  (Abundance x m/z)

                                        Total  Abundance          

           

RAM  =  (95.02 x 32)  +  (0.76 x 33)  +  (4.22 x 34)

                                                    (95.02  +  0.76  +  4.22)

 

                        RAM  =  32.09    (2dp as question is to 2dp)

 

2)  Boron has 2 isotopes and the mass spectrum is shown below.  Calculate its relative atomic mass:

 

                        RAM  =  (1 x 10)  +  (4 x 11)

                                                 (1  +  4)

 

                        RAM  =  10.8

 

3)   Lithium has two naturally occurring isotopes 6Li and 7Li. Calculate the percentage abundance of each isotopes given that the Ar of Lithium is 6.9.  Show your working.

            6.9 is 9/10 th's between 6Li and 7Li

            So 90 % must be 7Li so 10 % must be 6Li

            OR:

RAM = S  (Abundance x m/z)

                                        Total  Abundance          

           

6.9  =  (A x 6)  +  ((100-A) x 7))               Where A is the Abundance of 6Li