Question

1 amu = $\dfrac{1}{{12}}$ the mass of one:
(A) C-12 atom
(B) N-atom
(C) O-atom
(D) H-atom

Answer 1

Hint: 1 amu is equal to $1.66 \times {10^{ - 24}}gm$. Scientists decided to derive the value of amu to the element that is the most important element of the organic compounds. The element that is used to standardise the value of amu is a non-metal.

Complete step by step solution:
In earlier eras of ${20^{th}}$ century, mass of one hydrogen atom was taken as standard but then the concept of atomic mass unit came.
- In modern chemistry the carbon-12 isotope means isotope of carbon atom which has molar atomic mass of 12gm is taken as a standard and its mass is taken as 12amu and masses of all the other atoms are decided relative to that mass.
-Here amu stands for atomic mass unit.
So, correct answer of this question is (A) C-12 atom

Additional Information:
Let’s obtain the actual value of 1 amu.
- 12gm is the weight of 1 mole of carbon atoms, so weight of one carbon-12 atom can be found by dividing weight of 1 mole of carbon-12 atoms by Avogadro number which is equal to $6.023 \times {10^{23}}$.
So, weight of one carbon-12 atom=$\dfrac{{{\text{Mass of one mole of carbon - 12 atoms}}}}{{{\text{Avogadro number}}}}$
Weight of one carbon-12 atom = $\dfrac{{12}}{{6.023 \times {{10}^{23}}}}$
weight of one carbon atom = $1.9924 \times {10^{ - 24}}gm$
Now it is said that $1amu = \dfrac{{{\text{Mass of one carbon - 12 atom}}}}{{12}}$
1amu = $1amu = \dfrac{{1.9924 \times {{10}^{ - 24}}}}{{12}}$
1 amu = $1.66 \times {10^{ - 24}}gm$

Note: Remember that hydrogen was used as a standard to measure the weight of atoms in earlier era, now it is not used. It does not have any relations with atomic mass unit. Do not consider that 1 amu is equal to 1 gm of a compound.