# Chemical Forums

## Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: Win,odd Dhamnekar on November 19, 2020, 08:46:19 AM

Title: What is the approximate molecular weight of liquid?
Post by: Win,odd Dhamnekar on November 19, 2020, 08:46:19 AM

Title: Re: What is the approximate molecular weight of liquid?
Post by: AWK on November 19, 2020, 09:02:12 AM
You have all the information you need to calculate the molar mass from the ideal gas law.
Title: Re: What is the approximate molecular weight of liquid?
Post by: Win,odd Dhamnekar on November 21, 2020, 10:17:36 AM
Hello,
Weight of the unknown liquid inside the bulb after completion of reaction is 50.1879 grams - 50.0230 grams =0.1649 grams.

Volume of the water= 102.7535 grams - 50.0230 grams = 52.7305 grams = volume of the vapour of liquid after its immersion in oil bath at 423.15 K( 150° C) and 0.980 bar.

Using ideal gas law equation PV=nRT, $\frac{0.980 bar*0.0527305 liters}{0.0831447 bar*liters/(mol*K)*423.15 K}=0.001468787 mol$

Molar  weight of the unknown liquid =0.1649 grams/0.001468787 mol= 112.269 grams/mol. Its molecular weight is 1.86427e-22 u.

Title: Re: What is the approximate molecular weight of liquid?
Post by: AWK on November 21, 2020, 12:51:26 PM
The approximate molecular weight - 112.
Title: Re: What is the approximate molecular weight of liquid?
Post by: Win,odd Dhamnekar on November 21, 2020, 02:31:49 PM
Definitions of molecular mass, molecular weight, molar mass and molar weight

Molecular mass (molecular weight) is the mass of one molecule of a substance and is expressed in the unified atomic mass units (u). (1 u is equal to 1/12 the mass of one atom of carbon-12)

Molar mass (molar weight) is the mass of one mole of a substance and is expressed in g/mol
Title: Re: What is the approximate molecular weight of liquid?
Post by: mjc123 on November 21, 2020, 03:00:05 PM
Quote
Molar  weight of the unknown liquid =0.1649 grams/0.001468787 mol= 112.269 grams/mol. Its molecular weight is 1.86427e-22 u.
No. You have divided the molar mass by NA, which gives you the molecular weight in g. What is the molecular weight in u?
Title: Re: What is the approximate molecular weight of liquid?
Post by: Win,odd Dhamnekar on November 21, 2020, 10:38:11 PM
Oh! You are correct. AWK's answer is correct. Thanks for pointing out my mistake.   The molecular weight in u 1s 112.168
Title: Re: What is the approximate molecular weight of liquid?
Post by: mjc123 on November 22, 2020, 06:22:35 AM
Remember: The molecular weight in u is numerically equal to the molar mass in g/mol. (This follows from the definitions.)