Lesson 6, Topic 9
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# Molecular Formula

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The molecular formula of an element or compound is the formula which shows the exact number of atoms present in one molecule of the element or compound.

### Relationship between Empirical and Molecular Formula

(Empirical formula) n   =  Relative molecular mass

n = The actual number of atoms present in the element or compound.

Relative molecular mass of Empirical formula = Relative molecular mass of the compound

Molecular formula  = n x (Empirical formula)

Example I

A hydrocarbon contains 80% carbon and 20% hydrogen.

(a) Find the Empirical Formula of the compound. (b) If the relative molecular mass is 30, find its molecular formula. (H =1, C = 12)

Solution:

(a)

Empirical Formula   =    CH3

(b) $$\scriptsize (CH_3)_n = 30$$

$$\scriptsize (12 \: + \: 1 \: \times \: 3)_n = 30$$

$$\scriptsize 15_n = 30$$

$$\scriptsize n = \normalsize \frac{30}{15}$$

$$\scriptsize n = 2$$

Molecular Formula  = (Empirical Formula)n

Molecular Formula   = (CH3)2

Molecular Formula   =  C2H6

Example II

Find the Empirical formula of a compound which on analysis yields the following as the reacting masses. 2.0g of carbon, 0.34g of hydrogen, and 2.67g of oxygen. From your results, find the molecular formula of the compound, if its relative molecular mass is 60. (C = 12, H = 1, O = 16)

Solution:

Empirical Formula = CH2O

$$\scriptsize (CH_2O)_n = 60$$

$$\scriptsize (12 \: + \: 1 \: \times \: 2 \: + \: 16)_n = 60$$

$$\scriptsize (30)_n = 60$$

$$\scriptsize n = \normalsize \frac{60}{30}$$

$$\scriptsize n = 2$$

Molecular Formula =  (CH2O)2

Molecular Formula = C2H4O2   or   CH3COOH

Theory Questions

1. Atoms of the same element are alike in every aspect but differ from atoms of other elements. This is one of the ideals of Dalton’s atomic theory:

(a) Which modern discovery rendered the ideal invalid

(b) How has the ideal been modified by the modern atomic theory

2. (a) An element T exists in two Isotopic forms in ratio 2:3. If the relative masses of the Isotopes are 15 and 17 respectively. Calculate the relative atomic mass of the Element T.

(b) Calculate the relative molecular mass (RMM) of CaSO4 [Ca=40, S=32, O=16]

3. Copy and complete the table below

4. (a) Define the relative atomic mass of an element

(b) Calculate the relative molecular mass of

(i) Slaked lime Ca(OH)2

(ii) Sodium trioxonitrate(V), NaNO3

(iii) Copper(II)tetraoxosulphate. CuSO

(iv) Pentahydrate,CuSO4.5H2O

[Ca=40, O=16, H=1, Na=23, N=14, Cu=635,S=32]

5. a (i) Which instrument can be used to measure the mass of an atom of an element

(ii)Who invented the instrument mentioned in 5a (i) above.

b (i) What are Isotopes?

(ii) Name two elements that exhibits Isotopy, and give their Isotopic forms.

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