About molecular formula....

[albertlee]

How to find the molecular formula of the below question??

 

A gas with 87.5% of Carbon, and the remainning is Hydrogen, if the gas is 56g/mol, then what is its molecular formula??

 

 

Can you guys also show me the calculation??

 

thx in advance

 

Albert

[mak10]

1. C = 87.5

therefore, H = 100-87.5 = 12.5

 

2. divide the percentages by their atomic masses... so C = 7.29 and H = 12.5 itself.

 

3. get the ratio of the two.. 7.29:12.5 = 1:1.7 = 1:2 (rounded off)

 

4. thus, empirical formula is CH2.... now get its molecular mass which is 12 + 2 = 14.

 

5. divide 56 by 14 = 4 = n

 

6. therefore, (CH2) x 4 = C4H8 <=== and you have the molecular formula.

 

dont get too bogged down by the number of steps here... i just did it on purpose so you could understand every part of the calculation... its really simple once you get hold of the concept. just remember to get the empirical formula first by the usual calculations... then determine the value of n which is equal to the RMM of the given molecular formula divided by the RMM of the empirical formula you have found (it MUST be an integer!) and then simply do step 6.

 

-mak10

[albertlee]

How about it is 5.6g/mol, instead of 56g/mol???

 

Albert

[jdurg]

How about it is 5.6g/mol' date=' instead of 56g/mol???

 

Albert[/quote']

 

 

If the molar mass of the gas is 5.6 g/mol, then there is absolute no way that it can exist if it's 87% carbon. Carbon is ~12 g/mol, so ANY gas with carbon in it must be greater than 12 g/mol.

[Primarygun]

A gas with 87.5% of Carbon, and the remainning is Hydrogen,

Did he mean 87.5% of the number of particles or 87.5% of the mass of the particles?

[budullewraagh]

it's 87.5% of the mass, most certainly.

[Primarygun]

OK.

In reality, how do we know a new substance's molar mass?