If two elements B and C react with the
same mass of a third element (A), the ratio in which they do so will be the
same or simple multiple if B and C reacts with each other.

The above law is the basis of law of
equivalent masses, which will be described latter in details.

**For example:**

Ratio of masses of carbon and sulphur
which combine with fixed mass (32 parts) of oxygen is

**12:32 or 3:8**… (1)

In CS

_{2}ratio of masses of carbon and sulphur is**12: 64 or 3:16**… (2)
The two ratios (1) and (2) are related
to each other by

**3/8:3/18 or 2:1**

**ILLUSTRATIVE EXAMPLE (11):**One part of an element A combines with two parts of B (another element). Six parts of element C combine with four parts of element B. If A and C combines together, the ratio of their masses will be governed by:

(A) Law of
definite proportions (B) law of multiple proportions

(C) Law of
reciprocal proportions (D) law of conservation of mass

**SOLUTION**:

From this when

**A**combined with**C**the ratio is**A/C = 1/3**Hence**(C)**is correct.**ILLUSTRATIVE EXAMPLE (12):**Hydrogen combines with chlorine to form HCl. It also combines with sodium to form

**NaH**. If sodium and chlorine also combine with each other, they will do so in the ratio of their masses as

(A) 23:
35.5 (B) 35.5: 23

(C) 1: 1 (D)
23: 1

**SOLUTION**: 1 atom of hydrogen combined with 1 atom of chlorine and 1 atom of sodium. So when sodium reacts with chlorine the ratio is 1: 1 by atom. While 23: 35.5 by mass. Hence

**(A)**is correct.

**ILLUSTRATIVE EXAMPLE (13):**Copper sulphide contains 66.6% Cu, copper oxide contains 79.9% copper and sulphur trioxide contains 40% Sulphur. Show that these data illustrates law of reciprocal proportions.

**SOLUTION**:

In copper sulphides Cu: S mass ratio is 66.6: 33.4

In sulphur trioxide Oxygen: Sulphur (O: S) mass ratio is 60:40

Now in CuS 33.4 parts of sulphur combines with Cu = 66.6
parts

40.0 parts of sulphur combines with cu=66.68*40/33.4=79.8
parts

Now the ratio of masses of Cu and O
which combines with same mass (40 parts) of sulphur separately is 79.8:60 …
(1)

Cu: O ratio by mass in CuO is 79.9:
20.1 … 2)

Ratio
1: Ratio 2 = 1:3

Which is simple whole number ratio;
hence law of reciprocal proportion is proved.

**TRY YOURSELF:**

**Exercise 9.**Carbon combines with hydrogen to form three compounds A, B and C. The percentage of hydrogen in A, B and C are 25, 14.3 and 7.7 respectively. Which law of chemical combination is illustrated?

**Exercise 10.**61.8g of A combines with 80 g of B. 30.9g of A combines with 106.5g of C, B and C combine to form compound CB

_{2}. Atomic weight of C and B are respectively 35.5 and 6.6. Show that the law of reciprocal proportion is obeyed.

**Exercise 11.**Carbon dioxide contains 27.27% carbon, carbon disulphide contains 15.97% carbon and sulphur dioxide contains 50% sulphur. Show that these figures illustrate the law of reciprocal proportions.

*`***Exercise 12.**Aluminium oxide contains 52.9% aluminium and carbon dioxide contains 27.27% carbon. Assuming the validity of the law of reciprocal proportions, calculate the percentage of aluminium in aluminium carbide.

Answer of question number 12 please

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