Question

The percentage composition of Pb in $[P{b_3}{(P{O_4})_3}]$ is : (Given molar mass of $[P{b_3}{(P{O_4})_3}]$=811.0 gm mol)
A.73.4
B.76.5
C.80.1
D.68.54

Answer 1

Hint:
To calculate the percentage composition we need to find the mass of component and total mass of mixture, they are then divided followed by a multiplication with 100. The Molar mass of Pb is 207 gm mol.

Complete step by step answer:
The percentage composition of any constituent of the compound in a given compound is nothing but the percentage of the total mass of the given compound, that is being contributed by the constituent in question.
In simpler terms, we can say that the percentage composition of any constituent is a way to determine the total contribution of the constituent to the total weight of the compound in a percentage form.

In order to calculate the percentage composition, we must follow the following steps:
We must find the molar mass of all elements in the given compound in gram per mole.
Then, we must find the molecular mass of the entire compound.
After this, we must divide the desired constituent’s molar mass by the entire molecular mass of the compound.
Following which, we must multiply the above number with the number of molecules of the constituent present in the compound.
Finally, multiply the above number with 100 to get the percentage composition of the constituent

If we were to put these steps in the form of a formula, we would get;
% Composition of ‘x’ = $\dfrac{{n \times {M_x}}}{{{M_{comp}}}} \times 100$
Where, n = number of molecules of ‘x’ in the given compound
           ${M_x}$ = Molar mass of ‘x’
      ${M_{comp}}$=Molar mass of the compound

Hence, using the above formula for our problem, we get;
% Composition of Pb = $\dfrac{{n \times {M_{Pb}}}}{{{M_{[P{b_3}{{(P{O_4})}_3}]}}}} \times 100$
                 = $\dfrac{{3 \times 207}}{{811}} \times 100 = \dfrac{{621}}{{811}} \times 100 = 76.5\% $

Hence, Option B is the correct answer.

Note:
Remember that, the sum of all mass percentages should be 100% when added up. It is also known as percent by weight.