Question

a pure gas that is 14.3% hydrogen and 85.7% carbon by mass has a density of 2.5g/L at 0 degree celsius and 1atm pressure.what is the molecular formula of gas?
a)ch2 b)c2h4 c)c4h8 d)c6h12

Answer 1

use formula ,
density = PM/RT

where P is the pressure , R is universal gas constant , M is molecular mass of gas , T is temperature of gas in Kelvin.

2.5 = 1 × M /273 × 0.082

M = 2.5 × 273 × 0.082 g/mol
= 55.965 ≈ 56 g/mol -------(1)

now,

step 2:- find emperical formula ,
in compound , H is 14.3℅
and C is 85.7 %
so,

divided by atomic mass of it
H ==== 14.3/1 = 14.3
C ====85.7/12 = 7.14

divide 7.14 to both H and C

H ===== 2
C =====1

so , empirical formula , = CH2
hence molecular formula, = n×CH2
So,
molecular mass = n×( 12 + 2) = 56
from equation one M = 56 g/mol

so, n× 14 = 56

n = 4

so, molecular formula = 4× CH2 = C4H8
option (C) is correct

Answer 2

Answer:

empirical formula

 

=

 

C

H

2

.

molecular formula

 

=

 

C

6

H

12

Explanation:

AS with all these problems, we assume (for simplicity)  

100

⋅

g

of unknown compound.

And thus there are  

85.7

%

×

100

⋅

g

12.011

⋅

g

⋅

m

o

l

−

1

with respect to carbon, i.e.  

7.14

⋅

m

o

l

⋅

C

And  

14.3

%

×

100

⋅

g

1.00794

⋅

g

⋅

m

o

l

−

1

with respect to hydrogen, i.e.  

14.1

⋅

m

o

l

⋅

H

.

And thus the  

empirical formula

, the simplest whole number ratio that defines constituent elements in a species is  

C

H

2

.

Now the  

molecular formula

is alway a whole number multiple of the  

empirical formula

:

empirical formula

×

n

 

=

 

molecular formula

So  

(

12.011

⋅

g

⋅

m

o

l

−

1

+

2

×

1.00794

⋅

g

⋅

m

o

l

−

1

)

×

n

=

84

⋅

g

⋅

m

o

l

−

1

.

So  

n

=

6

, and  

molecular formula

 

=

 

6

×

C

H

2

 

=

 

C

6

H

12

Explanation: