Question

a 0.580 g sample of a compound containing only carbon and hydrogen contains .480 g of carbon and .100g of hydrogen. At STP 33.6mL of the gas has a mass of .087g. What is the molecular formula for the compound? The answer should be C4H10. Please explain with steps. Thank you.

Answer 1

1st find the molar mass of gas:

Given:

P = 1.0 atm

V = 33.6 mL

= (33.6/1000) L

= 0.0336 L

T = 273.0 K

find number of moles using:

P * V = n*R*T

1 atm * 0.0336 L = n * 0.08206 atm.L/mol.K * 273 K

n = 1.5*10^-3 mol

mass(solute)= 0.087 g

use:

number of mol = mass / molar mass

1.5*10^-3 mol = (0.087 g)/molar mass

molar mass = 58.01 g/mol

Now find the molecular formula

we have mass of each elements as:

C: 0.48 g

H: 0.1 g

Divide by molar mass to get number of moles of each:

C: 0.48/12.01 = 0.04

H: 0.1/1.008 = 0.0992

Divide by smallest:

C: 0.04/0.04 = 1

H: 0.0992/0.04 = 2.5

Multiply by 2 to get simplest whole number ratio:

C: 1*2 = 2

H: 2.5*2 = 5

So empirical formula is:C2H5

Molar mass of C2H5,

MM = 2*MM(C) + 5*MM(H)

= 2*12.01 + 5*1.008

= 29.06 g/mol

Now we have:

Molar mass = 58.01 g/mol

Empirical formula mass = 29.06 g/mol

Multiplying factor = molar mass / empirical formula mass

= 58.01/29.06

= 2

So molecular formula is:C4H10

Answer: C4H10