How to find molecular formula if molecular weight is not mentioned in question?

Asked by Jayant | 21st Apr, 2013, 11:03: AM

Expert Answer:

t's very easy. First of all we test the given sample in laboratory to know about its constitution. Let's discuss it by taking an example:- 
Suppose the constitution of a compound is given as below:- 
C = 92.4%, H = 7.6% 
let's assume that the sample is 100 gram. so the weight of carbon is 92.4 gram & that of hydrogen is 7.6 gram. Now it's we must make sure that is the total of weights of constituents 100 or not? If it is not 100, it means there is an another element in the compound, and this is always oxygen in case of hydrocarbons. 

Step 1st :- Find out the number of moles of each element 

number of moles = weight of the element/atomic mass 

number of moles of C = 92.4/12 = 7.7 moles 

number of moles of H = 7.6/1 = 7.6 mole 

Step 2nd:- Find out the relative ratio of the moles:- It is simply done by dividing all by the smallest one 

C = 7.7/7.6 = 1.01 

H = 7.6/7.6 = 1 

Step 3rd:- If the ratio is not whole numbers the multiply all the numbers by a smallest integer number to make them whole numbers. 

Here in this case both are nearly whole numbers. so there is need to multiply these numbers.

C = 1 
H = 1 

Step 4th:- Write the empirical formula 


step 5th:- Find the weight of empirical formula 

1*12 + 1*1 = 13 

now if the molecular weight is given then the molecular formula can be written easily. 

molecular weight = ( empirical formula weight)n 

for example the molecular weight is 78. 

n = 78/13 = 6 

so the molecular formula is = C6H6

Answered by  | 22nd Apr, 2013, 04:57: AM

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