Law of Constant Composition (Definite Proportion)

Topic Content:

The Law of Constant Composition was suggested by Joseph Louis Proust, a French chemist, in 1799.

The law states that all pure samples of the same chemical compound contain the same elements combined in the same proportion by mass.

Verification of the Law:

This can be experimented with by preparing Copper(II) oxide in three different ways.

(a) Action of heat on Copper(II) trioxonitrate(V)

\( \scriptsize 2Cu(NO_3)_{2(s)} \: \rightarrow \: \underset {black}{2CuO_{(s)}} \: + \: 4NO_{2(g)} \: + \: O_{2(g)} \)

(b) Action of heat on Copper(II) trioxocarbonate(IV)

\( \scriptsize Cu(CO_3)_{(s)} \: \rightarrow \: CuO_{(s)} \: + \: CO_{2(g)} \)

(c) Action of heat on Copper(II) hydroxide

\( \scriptsize Cu(OH)_{2(s)} \: \rightarrow \: CuO_{(s)} \: + \: H_2O_{(g)} \)

Analysis of Three Samples of Copper(II) Oxide (CuO):

Determine the amount of copper present in three samples of copper(II) oxide by reducing the oxide in steam of hydrogen.

Verification of the Law of Constant Composition (Definite Proportion)

Procedure:

Weigh three metal boats. Add a reasonable amount of samples A, B and C in each of the boats, respectively. Reweigh and