Question

One a.m.u or one ‘u’ is equal to:
a.) 1.6605389210 x ${ 10 }^{ -27 }$ kg
b.) 1.6605389210 x ${ 10 }^{ -29 }$ kg
c.) 1.2345765610 x ${ 10 }^{ -27 }$ kg
d.) 1.2345765610 x ${ 10 }^{ -29 }$ kg

Answer 1

Hint: To answer this question we should know that one a.m.u. is equal to 1/12 of the mass of an atom of carbon-12. Now you have to use this hint to find the answer with the correct unit.

Complete step by step answer:

The mass of an atom in a.m.u. is roughly equal to the sum of the number of protons and neutrons in the nucleus.
One atomic mass unit (a.m.u.) is the mass of a proton or a neutron which is equal to 1.6605389210 $\times$ ${ 10 }^{ -27 }$ kg.
Let’s try to understand it with an example -
The mass of a Hydrogen atom is 1 a.m.u. But, also we can say that the molar mass of hydrogen is equal to 1 g/mol and this is because:
1.6605389210 x ${ 10 }^{ -24 }$ g × 6.022×${ 10 }^{ 23 }$ = 1.007 g/mol
which is close to the mass that you find in a periodic table.

Therefore, the correct answer to this question is option A.

Note: You should also know that the a.m.u. is used to express the relative masses and thereby differentiate between various isotopes of elements. Thus, for example, uranium-235 (U-235) has an a.m.u. of approximately 235, while uranium-238 (U-238) is slightly more massive. The difference results from the fact that U-238, the most abundant naturally occurring isotope of uranium, has three more neutrons than U-235, an isotope that has been used in nuclear reactors and atomic bombs.