Tue, 23/04/2013 - 15:56
#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
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