Question

A laboratory group measures an excess of a volatile liquid. They add this to an empty flask with a volume of 56 mL. They heat the sample to 83.4 degrees Celcius until all the liquid is vaporized. The pressure in the lab is 0.987 atm. If 0.242 grams of volatile liquid were left in the flask, what was the molar mass of the compound? Record your answer to 2 decimal spaces.

Answer 1

We can determine the molecular mass using Ideal gas equation

Temperature to which the sample is heated = 83.4⁰C

Converting temperature from degree celcius to kelvin by adding 273.15

T= 273.15 + 83.4 = 356.55 K

Mass of volatile liquid = 0.242 grams

Pressure = 0.987 atm

Volume = 56 ml

1 litre = 1000ml

Converting the volume in ml to litre

V=56 ml = 56/1000 litre = 0.056 L

R = 0.0821 lit-atm/mol - k
Using Ideal gas Equation PV = nRT we can determine the number of moles of a gas

n = PV/RT = 0.987 atm x 0.056 Lit/ 0.0821 lit-atm/mol - k x 356.55 k

n = 0.055272 / 29.272755 = 0.0018881721 moles

We will use the formula for number of moles to determine molecular weight

number of moles = weight of the substance / Molecular weight of the substance

n = 0.0018881721 = 0.242 grams / M

M = 0.242 / 0.0018881721 = 128.166 grams

Molecular weight of the substance = 128.166 grams = 128.17 grams