Question

X gm A atoms on combining with Y atoms of B form 5 molecules of a compound containing A and B. Find the molecular weight of compound formed.(Atomic weight of B=M)
(a) $\dfrac{XNA+MY}{5}$
(b) $\dfrac{X+M}{5}$
(c) $\dfrac{X+MY}{5}$
(d) $\dfrac{X+MYNA}{5}$

Answer 1

We have been given the mass of A atoms and the mass of the atoms and then by applying the formula as $weight\text{ }of\text{ }A+weight\text{ }of\text{ Y atoms of }B=weight\text{ }of\text{ 5 molecules }AB$, we can easily calculate the mass of the five molecules of the compound AB formed.

Complete step by step solution:
We have been given A and B combines to form together 5 molecules of AB.
The reaction is supposed to occur as;
$A+YB\to 5AB$
We can find the molecular weight of compound AB as;
$weight\text{ }of\text{ }A+weight\text{ }of\text{ Y atoms of }B=weight\text{ }of\text{ 5 molecules }AB$ -----------(1)
Weight of A= $X\text{ }gram$ (given)
Weight of B=$M\text{ }gram$ (given)
Since there are Y atoms of B , so therefore the weight of Y atoms of B=$YM\text{ gram}$
Let suppose weight of AB= $W\text{ gram }$
Since, there are five molecules of AB formed, therefore,
Weight of 5 molecules of AB=$5W$
Now put all , these values in equation (1), we get;
$\begin{aligned} & X+YM=5W \\\ & W=\dfrac{X+YM}{5}\text{ grams} \\\ \end{aligned}$
Thus, when X gm A atoms combine with Y atoms of B to form 5 molecules of a compound containing A and B, then the molecular weight of the compound is $\dfrac{X+MY}{5}$.

Hence, option (c) is correct.

Note: Don’t get confused in the molecular weight and atomic weight. By the molecular weight we mean the sum total of the atomic weights of all the atoms present in that very compound . It is measured in Daltons or a.m.u i.e. atomic mass units. On the other hand, by the atomic weight , we mean the relative mass of the atom.