Composition of a Compound

A compound contains two or more elements combined in a certain fixed ratio.

The composition of a compound can be described by the mass percentage of each element present in it.

This may be determined by any of the methods described below.

 

Determining the composition of a compound

Composition of a compound is commonly expressed as the mass percentage of each element present in it.

To determine the mass percentage of an element in any compound, we should know the mass of that element present in a certain known mass of the compound.

Let us suppose, W g of a compound contain w g of an element A.

Then,

To do this, a known mass of the compound is chemically analysed, and the mass of each element present in it is determined by a suitable chemical method.

EXAMPLE 1.

5.85 g of a compound contains 3.55 g of chlorine, and 2.30 g of sodium. Determine the mass percentage of sodium and chlorine in the compound.

Solution: Mass of the compound = 5.85 g

       Mass of sodium = 2.30 g

       Mass of chlorine = 3.55 g

Then, 

  

Thus, mass percentage of sodium and chlorine in the given compound are 39.3 and 60.7, respectively.

Percentage composition of a compound determined from its formula

The percentage composition of a compound means the mass percentage of each element present in the compound. The mass percentage of each element can be determined from the molecular or empirical formula of the compound as follows.

Step 1. Write the molecular or empirical formula of the compound.

Step 2. Write the name, number of atoms, and the total mass of each element present in one molecule (or formula) of the compound.

Step 3. Obtain the molecular mass or formula mass of the compound by adding the masses of all the atoms present in it.

Step 4. Calculate the mass percentage of each element by using the relationship:

This method of calculating the percentage composition of a compound from its molecular formula is illustrated through the following examples:

EXAMPLE 5.2

Calculate the mass percentage of each element present in water (H₂O).

Solution: The molecular formula of water is H₂O.

A molecule of water is made up of two hydrogen atoms and one oxygen atom. Thus, one can write,

Molecular formula of water: H₂O = 2 atoms of H + 1 atom of O

   Atomic mass of H = 1 u      and   Atomic mass of O = 16 u

Therefore, Mass of two H atoms = 2 × 1 = 2 u

       Mass of one O atom = 1 × 16 = 16 u

and     Molecular mass of water = 2 u + 16 u = 18 u

So,

 

Therefore, water (H₂O) contains 11.11% of hydrogen (H), and 88.89% of oxygen (O).

EXAMPLE 5.3

Calculate the mass percentage of oxygen in (a) HgO  (b) K₂Cr₂O₇ (c) Al₂(SO₄)₃.
The atomic masses (in u units) are: Hg=200.6, O=16, K=39, Cr=52, Al=27, S=32

Solution: (a) HgO = 1 atom of Hg + 1 atom of O

So, Molecular mass of HgO = (200.6 u + 16 u) = 216.6 u

Therefore,

Thus, HgO contains 7.39% oxygen.

 (b) K₂Cr₂O₇ = 2 atoms of K + 2 atoms of Cr + 7 atoms of O

        2 × 39 u       2 × 52 u           7 × 16 u

                          78 u            104 u               112 u

So, Molecular mass of K₂Cr₂O₇ = 78 u + 104 u + 112 u = 294 u

Thus, 294 units of K₂Cr₂O₇ contain 112 units of oxygen. Therefore,

Thus, K₂Cr₂O₇ contains 38.1% oxygen.

 (c) Al₂(SO₄)₃ = Al₂S₃O₁₂ = 2 atoms of Al + 3 atoms of S + 12 atoms of O

                                        2 × 27 u         3 × 32 u         12 × 16 u

                                          54 u                96 u             192 u

So, Molecular mass of Al₂(SO₄)₃ = (54 + 96 + 192) u = 342 u

Thus, 342 u of Al₂(SO₄)₃ contain 192 u of oxygen (O).

Therefore,

Thus, Al₂(SO₄)₃ contains 56.1% of oxygen.

EXAMPLE 5.4

How many grams of Cr are there in 85 g of Cr₂S₃?       Atomic masses are: Cr = 52 u and S = 32 u.

Solution: Cr₂S₃ = 2 atoms of Cr + 3 atoms of S

                         2 × 52 u            3 × 32 u

                           104 u                96 u

        Molecular mass of Cr₂S₃ = (104 + 96) u = 200 u

So,      200 u of Cr₂S₃ contain = 104 u of Cr

or       200 g of Cr₂S₃ contain = 104 g of Cr

Thus,

Do It Yourself

1.   Calculate the percentage of sulfur in sodium sulfate, Na₂SO₄.

2.  Calculate the percentage of iron in the common ore, haematite, Fe₂O₃.

3.   Calculate the percentage of barium in barium hydroxide, Ba(OH)₂.

4.   Calculate the percentage of each element in:

                  a. silver nitrate, AgNO₃                b. ammonium phosphate, (NH₄)₃PO₄

5.   Calculate the percentage of nitrogen in:

           a.   ammonium sulfate, (NH₄)₂SO₄   b. urea, CH₄ON₂

6. A popular explosive has the molecular formula, C₇H₅N₃O₆. Calculate the percentage of nitrogen in the compound.

[Atomic masses: H = 1.0 u, C = 12.0 u, N = 14.0 u, O = 16.0 u, P = 31.0 u,
S = 32.0 u, Na = 23.0 u, Fe = 56.0 u, Ba = 137.0 u and Ag = 108.0 u]