An organic chemist has produced a solid she believes to be...

An organic chemist has produced a solid she believes to be pure; she expects a molar mass of $346 \mathrm{g} / \mathrm{mol}$. Using 4.08 grams of the solid, she melts and then boils it in a 3.36 -L vacuum chamber at 117 torr and $243^{\circ} \mathrm{C}$. She is disappointed; the molar mass is close to what she expected, but not close enough. (a) What molar mass did she find? (b) Her finding suggested to her what her mistake might have been. Does it suggest anything to you? [Part (b) is beyond the scope of Chapter $14,$ but you might have an inspiration if you have studied Chapter 13.]

Key Concepts

Chemical Decomposition and Reaction

If a compound decomposes or undergoes chemical reaction upon heating, the gas collected may not represent the intact parent molecule. Such decomposition can result in fragments or by-products that lead to an apparent molar mass differing from the expected value, thereby signaling potential issues in the experimental procedure or the stability of the compound.

Phase Transitions and Vaporization

Understanding phase transitions—especially vaporization—is important when a solid is heated to form a gas for analysis. Experimentally, conditions such as vacuum and temperature are manipulated to induce sublimation or boiling. The behavior during these transitions can affect the accuracy of molar mass determinations if the substance alters chemically or physically.

Ideal Gas Law

The Ideal Gas Law (PV = nRT) is a core principle in physical chemistry that relates the pressure, volume, and temperature of a gas to the number of moles present. It is essential in experiments requiring the determination of the number of moles within a gas sample from measurable macroscopic properties, which in turn is used for calculating molecular weight.

Molar Mass Determination

Molar mass determination by vapor density involves measuring the mass of a vapor along with its pressure, volume, and temperature, then using the Ideal Gas Law to calculate the number of moles. Dividing the mass by the number of moles gives the molar mass; discrepancies in the expected and measured values can indicate experimental or chemical issues.

Transcript

00:01 Hi there.

00:02 In this problem, an organic chemist creates a compound, and she expects this compound to have a molar mass of 346 grams per mole.

00:26 After doing some experimentation, she realizes that is not the molar mass of this compound.

00:34 And from her data, we are asked to calculate the molar mass of the actual compound that she made.

00:46 So i'm going to use the equation.

00:48 Molar mass is equal to mass times the universal gas constant times the temperature divided by pressure times volume.

00:59 Based upon what we were given in this problem, we were given a sample mass of 4 .08 grams.

01:14 Our pressure is measured in tor.

01:16 So i am going to use the universal gas constant that has tor as one of the units.

01:21 That's 62 .4 liters times tor over mole times kelvin.

01:32 And the temperature we're given in the problem is 243 degrees celsius.

01:38 Knowing that in gas law problems of any type, we need to have the temperature in kelvin.

01:45 So i added 273 to 243, and we have a kelvin temperature of 516.

01:57 We're going to divide this by p times v.

02:01 The pressure is 117 tor.

02:08 And the volume that we are given, the volume of that flask that she used, was 3 .36 liters...

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