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# A gaseous compound containing hydrogen and carbon is ## Problem 104P Chapter 11

Introductory Chemistry | 5th Edition

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Problem 104P

PROBLEM 104P

A gaseous compound containing hydrogen and carbon is decomposed and found to contain 85.63% C and 14.37% H by mass. The mass of 258 mL of the gas, measured at STP, is 0.646 g. What is the molecular formula of the compound?

Step-by-Step Solution:

Solution 104P

Here, we are going to find the molecular formula of the given compound.

Step 1:

Molecular formula gives the total number of atoms in a molecule.

Step 2:

In order to determine the whole number ratio of C and H in the formula, the following table is drawn:

 Element % Relative number of atoms(% / atomic mass) Simple ratio Whole number ratio C 85.63 85.63/12 = 7.14 7.14/7.14 = 1 1 H 14.37 14.37/1 = 14.37 14.37/7.14 = 2.01 2

Hence, empirical formula of the compound is CH2.

Empirical formula mass = (1 x 12) + (2 x 1) = 12 + 2 = 14

Step 3:

Again, from ideal gas equation, we have,

pV = nRT        ----(1)

Where, p = pressure

V = volume of the gas

n =  number of moles of the gas

R =  universal gas constant and its value is 0.0821 L atm K-1 mol-1

T =  temperature

Here, p = 1 atm (STP)

V = 258 mL = 258 / 1000 L = 0.258 L

T =  273 K (STP)

Substituting the values in equation (1), we get,

n = pV / RT

n = (1 atm x 0.258 L) / (273 K x 0.0821 L atm K-1 mol-1)

= (0.258 L atm) / (22.4133 L atm mol-1)

n = 0.0115 mol.

Step 4 of 6

Step 5 of 6

##### ISBN: 9780321910295

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