Question

a) Define law of constant proportion.
b) Define molality. Why is it superior over molality?
c) How are $0.50{Na}_2{CO}_3$ and $0.50M{Na}_2{CO}_3$ different?

Answer 1

Solution:-

(a) Law of constant proportion $\to$ It states that "a pure chemical compound always consists of the same elements that are combined together in a fixed proportion by mass".

(b) Molarity $\to$ It is the concentration of a solution expressed as the number of moles of solute per litre of solution.

Molarity is the number of moles per unit volume of the solution and molality is the number of moles per unit mass of solvent. Volume is temperature dependent whereas mass is constant at all temperatures. Thus, molality remains constant but molarity changes with temperature. Hence, molality is superior over molarity.

(c) Molar mass of ${Na}_{2}C{O}_3=106g/mol$

Therefore,

$0.5$ moles of ${Na}_{2}C{O}_3$ means $0.5\times 106=53g$

$0.50M{Na}_{2}C{O}_3$ means $0.50$ moles of ${Na}_{2}C{O}_3$, i.e., $53g$ of ${Na}_{2}C{O}_3$ are present in $1L$ of the solution.