Question

A meal sulphide (MS) which is responsible for the discoloration of paints contains 13.3 percent sulphur by mass. Identify the metal likely to be present in the sulphide.

Answer 1

Mass percent is the mass of an element or a solute divided by the mass of the compound or the solute. After this the result is multiplied by 100 to convert it into percent. The formulae for the calculation of mass percent i.e. the amount of element present in a compound is:
Mass percent = $\dfrac{\text{mass of element in one mole of compound}}{\text{mass of 1 mole of a compound}}(100)$

Complete answer:
Mass percent will be equal to the ratio of the mass of element in one mole of compound to the mass of 1 mole of compound multiplied by 100.
Let the atomic weight of the metal be x
We know that the atomic weight of sulphur is = 32 g or 32 amu
So the molecular weight of the metal will be the sum of atomic weight of metal x and atomic weight of sulphur
Molecular weight of metal= $32+x$
But given in the question is the metal contains contain 13.3 percent sulphur by mass
So it will be 13.3 percent of 32
So the atomic weight of the other metal is =
$(32+x)(13.3$
$32+x=\dfrac{(32)(100)}{13.3}$
The atomic weight of the metal is = x = 208.60
208.60 is approximately equal to the atomic weight of Bismuth
The metal which is likely to be present is bismuth.

Note:
Do not get confused between the atomic and the molecular mass, molecular mass is the sum of the atomic masses of an atom or a molecule. Different molecules of the same compounds may have different molecular masses because they can contain different isotopes of an element.