1.1   ATOMIC STRUCTURE

Mass Spectrometry

Section A

 

For each of the questions only one of the lettered responses (A-D) is correct.

 

Select the correct response in each case and mark its code letter by connecting the dots as illustrated on the answer sheet.

 

1.              Using a mass spectrometer, it is possible to determine the number of

              A  protons in an atom

              B   energy levels in an atom

              C   isotopes of an element

              D  neutrons in an atom

 

2.             The accurate relative isotopic masses of five isotopes are

11H = 1.0078      21H = 2.0141   126C = 12.000  147N = 14.0031    168O = 15.9949

Using a high resolution mass spectrometer, a certain gas was found to have a relative molecular mass of 28.0172. The gas could be            

     A  147N2

     B   126C2 11H4

     C   21H 126C 147N

     D  126C2 21H2

 

3.             The mass spectrometer trace for naturally occurring magnesium is shown below. Assuming that all three peaks relate to ions with one positive charge, what is the relative atomic mass for magnesium?

          A  24.2

          B   24.3

          C   24.4

          D  24.7

 

 

 

 

 

 

4.       The isotopic composition of a certain element X is 80% 24X, 10% 25X and 10% 26X.  The relative-atomic mass of X is

              A  24.25

              B   24.30

              C   24.33

              D 24.67

 

5.             The diagram below shows the mass spectrometer trace of the substance X in the region of relative mass 10 to 20 units.

 

 

 

 

 

 

 

 

 

Which of the following substances is X most likely to be?

     A  O2

     B   NH3

     C   H2O

     D  CH4

 

6.    Which one of the following could not be obtained using a mass spectrometer?

            A    The number of electrons in an isotope of manganese

            B     The number of isotopes present in a sample of chlorine

            C     The RMM of a sample of cocaine

            D    The mass of an isotope of uranium

 


9     The mass spectrum of an element is as shown below.

 


Assuming each fragment has unit charge, what is the relative atomic mass of the element?

       A    206.25

       B     207.00

       C     207.77

       D    208.00

 

           


SECTION B

1.              The mass spectrum of naturally occurring rubidium, Rb, is shown below.

From this spectrum calculate the relative atomic mass of rubidium.

 


            100

 

 

 

               55

 

 

 

                 0
                           m/z                 85         87

 

 

 

 

                                                                                                                           [3]

Ar = 100 x 85 + 55 x 87 = 8500 + 4785 = 13285 = 85.71

        155        155      155

 

2.

(a)     An organic compound has the composition by mass.

                  

Element

Percentage

Hydrogen

4.1

Carbon

24.2

Chlorine

71.7

 

(i)      Calculate the empirical formula of the compound.

 

 

 

 

 

 

 

(ii)     The mass spectrum of the compound showed the molecular ion peak at a mass/charge ratio of 98. Suggest a structural formula for the compound.

 

 

 

 

 

 

(iii)    Suggest why smaller peaks may be found at 100 and 102.

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       [2]

3.       A mixture of 21H2 and 8135Br2 was analysed in a mass spectrometer. The following pattern of lines due to singly-charged ions was obtained.





        2       4                                  81       83                                      162

                                       mass number on instrument scale

 

          Suggest which ions give rise to each of these lines.

                       

Mass number

Ion responsible

2
21H+

4
21H2+

81
8135Br+

83
21H8135Br+

162
8135Br2+

                                                                                                                                    [3]

 

 

4. Beryllium occurs to a small extent in the earth's crust.  It is a steel-grey metal, which is extremely light. Beryllium is found in nature as 9Be.

 

(i) How could you show that naturally occurring beryllium consisted only of 9Be?

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       [2]

 

(ii)   Draw the structure of 9Be in terms of the constituent particles of the atom.

 

 

 

 

 

 

 

                                                                                                                                    [2]

 

4.    (a)  Complete the table below to show the relative masses and charges of a proton, a neutron and an electron.                                                                       [3]

 

 

Relative mass

Relative charge

Proton

 

 

Electron

 

 

Neutron

 

 

 

(b)    Describe the process by which particles are ionised in a mass spectrometer.                                                            [2]

………………………………………………………………………………………………………………………………………

………………………………………………………………………………………………………………………………………

 

(c)    Give two reasons why particles must be ionised before being analysed in a mass spectrometer.                                                     [2]

Reason 1  ……………………………………………………………………………………………………………………

Reason 2  ………………………………………………………………………………………………………………….

 

(d)    A sample of boron contains 20% by mass of 10B and 80% by mass of 11B. Calculate the relative atomic mass of boron in this sample.                      [2]

………………………………………………………………………………………………………………………………………

………………………………………………………………………………………………………………………………………

………………………………………………………………………………………………………………………………………

 

(e)    Compound X contains only boron and hydrogen. The percentage by mass of boron in X is 81.2%. in the mass spectrum of X the peak at the largest value of m/z occurs at 54.

(i)         Use the percentage by mass data to calculate the empirical formula of X.                                                                                              [3]

………………………………………………………………………………………………………………………………………

………………………………………………………………………………………………………………………………………

………………………………………………………………………………………………………………………………………

(ii)       Deduce the molecular formula of X.                                                         [1]

………………………………………………………………………………………………………………………………………

                                                                                                                  AQA