Question

On analysis, a substance was found to have the following percentage composition: A = 32, B = 29 and O = 39 (At. mass of A=39 and B=35). The molecular mass of the substance is 244. Its molecular formula is

Answer 1

I will tell you the simplest trick to solve these problems for both school exams and MCQs.

• First look at the diagram and prepare a similar table.

• In the two rows, write element name and atomic mass as given in question.

• Then calculate value of Given Mass/Atomic mass in 3rd row.

• Finally try to find the simplest ratios between A, B and O.

The ratio came as 2 : 2 : 3.

So, empirical formula is :

$\boxed{A_{2}B_{2}O_{3}}$

Now,

$\rm \implies \:( 39 \times 2) + (35 \times 2) \times (32 \times 3) = n \times 244$

$\rm \implies \:244 = n \times 244$

$\rm \implies n = 1$

So, molecular formula is also :

$\boxed{ \bf \: A_{2}B_{2}O_{3}}$