Question

A compound AB completely decomposes into A and B on heating. 50 g of AB on strong heating, gave 40 g of A. How much quantity of AB should be decomposed by heating to obtain 2.5 g of B? How much quantity of A will be produced in the process?

Answer 1

Hint: decomposition is the type of reaction where any compound undergoes a change in the presence of heat to decompose or convert into its constituent element form. These types of reactions involve one compound giving two products on decomposition.

Complete answer:
The law of conservation of mass states that the mass of reactants is always equal to the mass of products. The total mass is therefore conserved. So, the decomposition reaction will be,
$AB\to A+B$ , we have mass of AB to be 50 g and that of A in the products to be 40 g, therefore mass of B in the products will be 10 g. So, 50 g of AB gives A as 40 g and B as 10 g on decomposition. This makes B 20 % of AB on decomposition.
Therefore, we will get 2.5 g of B which is 20 % from the total amount of AB as,
AB when B is 2.5 g = $\dfrac{2.5\times 100}{20}$
AB when B is 2.5 g = 12.5 g
So, 12.5 g of AB will decompose to give 2.5 g of B. amount of A in this process is,
Amount of A = 12.5 – 2.5
Amount of A = 10 g
So, 12.5 g of AB decomposes to give 2.5 g of B and A is 10 g in the process.

Note:
To calculate the amount of AB from 2.5 g of B we put the 20 % of B into the calculation. As decomposition of AB yields 20 % of B from the given data. So we put 2.5 to be 20 % of x, where x is the amount of AB, as $2.5=\dfrac{20}{100}\times x$.