Volatile & Nonvolatile Solutes

Volatile & Nonvolatile Solutes
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  • 0:04 Non-Volatile vs.…
  • 1:22 Volatility,…
  • 1:35 Raoult's Law
  • 2:50 An Example Problem
  • 5:49 Important Points to Remember
  • 6:43 Lesson Summary
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Lesson Transcript
Instructor: Jennifer Perone

Ms. Perone has taught College Engineering, Ethics, Psychology, Perception, Statistics, Experimental Design & Analysis, Physics and secondary STEM topics for more than 15 years!

In this lesson, we'll cover the key differences between volatile and non-volatile substances while learning about vapor pressure and Raoult's law along the way.

Non-Volatile vs. Volatile Solutes

A solvent is something that a solute is dissolved into. For example, if you remember that water is the universal solvent, it will be easy to remember which part of a mixture is the solvent and that the solute is something like salt or sugar that is dissolved into it. Solution = solvent + solute.

A non-volatile substance refers to a substance that does not readily evaporate into a gas under existing conditions. Non-volatile substances exhibit a low vapor pressure and a high boiling point. Sugar and salt are examples of non-volatile solutes.

A volatile substance is one that evaporates or sublimates at room temperature or below. Volatile substances have higher vapor pressures versus non-volatile substances at the same temperature. Examples of volatile substances include alcohol, mercury, and gasoline.

A good way to remember which substances are volatile and non-volatile is to think about which types of substances you readily smell. You know when gasoline is around because it permeates its surroundings and this is because it readily releases its molecules into the air. This means gasoline is volatile. You have to either taste or smell a sugar or salt solution to determine which it is. You would not try to directly smell or taste gasoline. You know what it is from a distance.

Volatility, Temperature & Pressure

If you have a volatile substance, it will have a high vapor pressure and a low boiling point. An increase in temperature will cause an increase in vapor pressure, or the pressure at which the gas phase is in equilibrium with the liquid or solid phase.

Raoult's Law

Raoult's law is a thermodynamic law that states that the partial vapor pressure of each component of an ideal mixture of liquids equals the vapor pressure of the pure component multiplied by the molar fraction of the mixture.

Raoult's law is expressed by the vapor pressure equation:

Psolution = Xsolvent * Posolvent

  • Psolution = vapor pressure of the solution
  • Xsolvent = molar fraction of the solvent
  • Posolvent = vapor pressure of the pure solvent

If you're working with two or more solutions, you can just add the vapor pressure or each solution to find the total vapor pressure:

PTotal = Psolution 1 + Psolution 2 + and so on

What do all of these equations tell us about volatile versus non-volatile solutions? When you have two volatile solutions together, they will push each other toward evaporation and have high vapor pressures and low boiling points. When you have a non-volatile solution, such as a sucrose or salt solution, you have an increasing boiling point as the concentration of sucrose or sugar increases. If you have water plus a volatile substance, it will be pushed further and further towards evaporation and higher vapor pressure as the amount of volatile solution increases and vice versa.

An Example Problem

Calculate the vapor pressure for a mixture of pure toluene (C7 H8) and pure methanol (CH3 OH), given the following information:

  • There are 60 grams of toluene and 40 grams of ethanol
  • Vapor pressure of pure toluene at 20o C is 22 torr
  • Vapor pressure of pure methanol at 20o C is 94 torr

Step 1: Determine Molecular Weights and Molar Ratios of Each Substance

The first step is to determine the number of moles for each solution. You'll need this to calculate the mole fraction of components.

Carbon = 12 g/mol, hydrogen = 1 g/mol, and oxygen = 16 g/mol. Use the molecular weights to find the number of moles of each component. Toluene contains 7 carbons and 8 hydrogens, and the molecular weight will be:

  • 7(12 g/mol) + 8(1 g/mol) = 84 g/mol + 8 g/mole = 92 g/mol

Next, take the given amount of toluene (60 grams) and divide it by 92 grams/mol to obtain 0.652 moles. Check this on your calculator!

Methanol contains 1 carbon, 4 hydrogens, and 1 oxygen. Using the same process to find the molar weight of methanol, we have:

  • 1(12 g/mol) +1(16 g/mol) + 4(1 g/mol) = 32 g/mol

Next, take the given amount of methanol (40 grams) and divide it by 32 grams/mol to obtain 1.25 moles. Check this calculation as well. You've got to be careful with your figures!

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