Question

1: a) what is the mass in amu of a carbon 12 atom? b) why is the atomic weight of carbon reported as 12.011 in the table of elements and the periodic table?

2: The mass sprectrum of H2 is taken under conditions that prevent decomposition into H atoms. The two naturally occuring isotopes of hydrogen are ^1 H (atomic mass = 1.00783 amu; abundance 99.9885%) and ^2 H (atomic mass = 2.01410 amu; abundance 0.0115%) a) How many peaks will the mass spectrum have? b) Give the relative atomic masses of each of these peaks. c) Which peak will be the largest and which will be the smallest?

Answer 1

1:a)  A better, more complete explanation (applied the chlorine) can be found by pisgachemist or myself in the link below.

One mistake everyone else answering your question so far has made is that they neglect to say that the actual masses of the isotopes are NOT whole numbers (except for C-12, which is DEFINED as a whole number). That's a mistake that many people make. The mass of a proton is CLOSE to 1 but, it's not 1 (1.0073 amu). The mass of a neutron is also close to 1, but it's not 1 (1.0087 amu). The mass of C-13 is not 13 (13.0033548378 amu).

The isotopic abundances of C-12 and C-13 are 98.93% and 1.07%, respectively.

The atomic mass of carbon is (0.9893)(12 amu) + (0.0107)(13.00336) = 12.0107 amu

2: The three peaks you'll see will be 1H-1H, 1H-2H, 2H-2H and their corresponding masses should be:

1H-1H:
2(1.00783) = 2.01566 amu

1H-2H:

(1.00783 + 2.01410) = 2.02193 amu

2H-2H:

2(2.01410) = 4.02820 amu

Cheers.