Need help with this chemistry question!! Please
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Joined: 23/04/2013 - 11:19
Posts: 1
Need help with this chemistry question!! Please

A hydrocarbon is know to to contain a benzene ring. It has relative molecular mass of 106 and has the following composition by mass: C 90.56%, H 9.44%. 

a) Use the data above to show that the empirical formula is C4H5

b) Deduce the molecular formula

Im really stuck on these 2 Qs.. If anyone knows how to do them, pls help me!Thanks

Joined: 24/04/2013 - 19:26
Posts: 5
Here u go 2 ways :)

They havent given the relative atomic mass so we use the relative molecular mass = 106 

             Composition by mass     Carbon=90.56*106/100= 95.9926   hydrogen=9.44*106/100=10.0064

Now you divide the answer by the Ar of the elements so Carbon = 95.9926/12 =8(rounded up from 7.9) 

Hydrogen = 10.0064/1 =10 (We always use whole numbers in ratios)     

Now the ratio of carbon to hydrogen is 8:10 simplify is 4:5 so C4H5

Or you can do this, find the moles of each element first assume you have 100 grams of the compound and calculate moles of C and H from the percentages:

90.56g of Carbon X 1 mol/12 g = 7.55 mol Carbon

9.44g of Hydrogen X 1 mol/1 g = 9.44 mol Hydrogen

Divide both of those by the smaller:

7.55 mol C/ 7.55 mol = 1 Carbon

9.44 mol H/ 7.55 mol = 1.25 Hydrogen

Multiply both by 4 to make the simplest whole numbers: (LCM) lowest common multiple

1 C X 4 = 4 C

1.25 H X 4 = 5 H

Empirical formula is C4H5, 

im not sure what deduce is stilll in year 10 :p