Question

If n atoms of elements A weigh 15g and 4n atoms of elements B whose atomic weight is 30u, weigh 45g, then determine the atomic weight of A:
(A) 15u
(B) 40u
(C) 45u
(D) 30u

Answer 1

Hint: Approach this question by the simple unitary method scheme and also try to understand the concept atomic masses (or weights) and atomic mass units. As we know that atomic weight is the ratio of the average mass of a chemical element’s atoms to some standard reference whereas an atomic mass unit is defined as a mass equal to one twelfth the mass of an atom of carbon-12.

Complete answer:
Let us first understand the basic definition of atomic masses and atomic mass unit (amu or u):-
- As we know that atomic weight (also known as relative atomic mass) is the ratio of the average mass of a chemical element’s atoms to some standard reference whereas an atomic mass unit is defined as a mass equal to one twelfth the mass of an atom of carbon-12.
-Calculation of atomic weight of element A:-
Let us suppose the atomic weight of element A = X u
It is given that ‘n’ atoms of element A weighs = 15g
So $n\times X=15g----eq(A)$
The values provide for element B are as follows:-
4n atoms of B element weigh 45g and each atom has an atomic weight of 30 u.
So $4n\times 30=45g$
=$120n=45$
=$n=\dfrac{3}{8}$
-Now, we can substitute the value of ‘n’ in eq(A) as follows:-
$n\times X=15g----eq(A)$
$\begin{align}
  & \Rightarrow \dfrac{3}{8}\times X=15 \\
 & \Rightarrow X=15\times \dfrac{8}{3} \\
 & \Rightarrow X=40u \\
\end{align}$

Therefore the atomic weight of element A is (B) 40u

Note:
Remember to convert the given units into the units you are asked for and check the options as well. Also while solving the numerical questions, write the terms along with its unit so that you can easily cancel them out and get the result in the desired unit term.