Question

The empirical formula of a compound is CH. At 200°C, 0.145 g of this compound occupies 97.2 mL at a pressure of 0.740 atm. What is the molecular formula of the compound?

Answer 1

Solution:-

You can work out the molar mass from the ideal gas equation

PV = nRT

where n = moles
We know that
moles = mass (m) / molar mass (M)

So we can sub this into the gas equation


PV = mRT/M

P = pressure = 0.74 atm
V = volume = 97.2 ml = 0.0972 L
T = temp in Kelvin = deg C + 273.15 = 473.15 K
m = mass = 0.145 g
M = molar mass = ? g/mol
R = gas constant = 0.082057 L atm mol^-1 K^-1

Solve for M

M = mRT / PV
= 0.145 g x 0.082057 x 473.15 / 0.74 atm x 0.0972 L
= 78.3 g/mol

To find the molecular formula, divide the molar mass by the mass of the empirical formula. This tells you how many times the empirical formula is repeated in the molecular formula.

empirical mass = 12.01 + 1.008 = 13.018 g/mol

78.3 g/mol / 13.018 g/mol
= 6

molecular formula is thus
6 x CH
= C6H6