Boiling point elevation

Boiling point represents one of our four colligative properties. 

Boiling point elevation formula

The equation for boiling point elevation is i multiplied by kb multiplied by m. 

i represents your Van’t Hoff factor. That’s just the number of ions your solute will breakup into. If your solute is ionic, it breaks up into ions. If your solute is covalent, then it doesn't break up into ions at all; it stays intact so i equals 1. 

If you wanna take a closer look at the Van’t Hoff factor, make sure you take a look at our chapter videos and concept videos and see how we approach calculating the Van’t Hoff factor based on a specific solute given. 

Now we’re going to say that the lower case m is molality, which represents the moles of solute divided by kilograms of solvent. 

Kb represents the boiling point constant of our solvent. 

Below, I’ve provided some common types of solvents. We have water, benzene, chloroform and ethanol. You’re not expected to memorize all of their normal boiling points as well as their boiling point constants. But for water, I would recommend you memorize those. You should know that pure water boils at 100°Celsius. All you really have to remember other than that is that its boiling point constant is 0.51°Celsuis/molality. 

How to calculate boiling point

How do we apply this formula to figuring out the boiling point of a solution? We’re going to say that the boiling point of a solution equals the boiling point of the pure solvent plus my change in boiling point. Let’s say that we found that our change in our boiling point was equal to 0.7°C once we’ve plugged these variables in. We know that water boils at a temperature of 100°C. When I add solute to the pure solvent, the boiling point goes up that’s why it’s called boiling point elevation; there’s an increase in boiling point. I would just add my change in my boiling point temperature so it equals 100.7°C. That’d be the new boiling point of my solution. 

Summary

Remember, besides boiling point, we also have freezing point, osmotic pressure and Raoult’s Law, which deals with vapor pressure, as our three other colligative properties. Remember, the colligative properties help to explain what happens to a pure solvent as I add solute to it gradually over time. It causes changes in these four colligative properties. Make sure you take a look at our chapter videos and concept videos dealing with the colligative properties and the common types of problems that usually appear when discussing these four different types of properties.