Which one of the following is molecular formula of hydrocarbon which contain 80% by mass of
carbon and 20% by mass of hydrogen?
A) CH4 B) C2H6 C) C3H6 D) C4H8
Answers
Answer:
b) C2H6
PLZZZ MARK AS BRAINLIEST
BECAZ IT WORTHS
Explanation:
As with all these problems, it is usually assumed that we have a
100
⋅
g
mass of unknown compound, and we work out the molar quantities:
And thus
moles of carbon
≡
80
⋅
g
12.011
⋅
g
⋅
m
o
l
−
1
=
6.66
⋅
m
o
l
.
And thus
moles of hydrogen
≡
20
⋅
g
1.008
⋅
g
⋅
m
o
l
−
1
=
19.8
⋅
m
o
l
.
We divide the molar quantities thru by the SMALLER molar quantity:
C
:
6.66
⋅
m
o
l
6.66
⋅
m
o
l
=
1
H
:
19.8
⋅
m
o
l
6.66
⋅
m
o
l
=
2.97
And thus the
empirical formula
=
C
H
3
And we know that the molecular formula is always a multiple of the empirical formula, and thus is terms of mass:
i.e.
molecular formula
=
n
×
empirical formula
But we have a molecular mass of
30
⋅
amu
So
30
⋅
amu
=
n
×
(
12.01
+
3
×
1.01
)
⋅
amu
Clearly,
n
=
2
, and the MOLECULAR FORMULA is
C
2
H
6
.
This is not a realistic problem, as few analysts would perform combustion on a liquid, and no analyst could perform combustion on a gas.