Colligative Properties of Water
What happens when we add salt to boiling water?
*Note- This dataset was collected by real students, but students can easily collect their own data for this activity and use the same questions with their own dataset. Learn about working with your own data in DataClassroom here.
Introduction
Solutions are homogeneous mixtures of solute and solvent. The solvent is the most abundant substance in a solution. In a liquid solution, the solvent does the dissolving. The solute is the other substance in a solution. In a liquid solution, the solute is dissolved. It is possible for a solution to have more than one solute - in fact, air is an example of a gaseous solution with multiple solutes - but a solution can have only one solvent.
The boiling point of a substance is the temperature at which the liquid form of that substance transitions to the gaseous state. When a substance boils, the particles gain enough energy to overcome their attractions to one another and escape as a gas. When an ionic substance boils, ionic bonds break and individual ions enter the gaseous state. When a covalent substance boils, intermolecular attractive forces (IMAFs) break and individual molecules enter the gaseous state.
Dissolving a solute in a solvent increases the boiling point of the solvent. This boiling point elevation is a colligative property, which means that it is dependent only on the amount of solute that is dissolved in the solvent. Keep in mind that when ionic solids are dissolved in water they break up into ions, a process called ionization. When added to a compatible solvent, NaCl breaks up into two ions, MgCl2 breaks up into three ions, and AlCl3 breaks up into four ions. The number of particles that a solute breaks into when dissolved is referred to as the van’t Hoff factor. When a polar covalent solute is dissolved in water or another polar solvent, its van’t Hoff factor is one.
When a person adds salt to water when cooking pasta, this is an example of colligative properties at work. Though people typically add salt in order to season the pasta, the addition of the salt makes the water boil at a slightly higher temperature.
The Dataset
The dataset in this virtual lab activity was collected by real students. The students created salt water solutions of five different concentrations, heated each solution to a boil, and recorded the boiling points of each solution. The dataset includes data from three different student groups.
In your analysis, you’ll examine the collected data of three different groups (A-C) that measured the boiling points of five different aqueous solutions of sodium chloride. You’ll use this data to create a graph and carry out some calculations designed to verify the experimental boiling point elevation factor (Kb) of water.
Variables
Student Group - This categorical variable indicates which of the three student groups recorded the observation.
Solution Concentration (molal, m) - This numeric variable is the solution concentration for each observation.
Boiling Point (°C) - This numeric variable is recording the temperature for each observation.
The Activity
1. This lab requires you to use NaCl solutions of various concentrations. Determine the correct amount of NaCl (in grams) needed to prepare 200. mL of each of the following solutions.
Note:
1 mL of water has a mass of 1 g
molality (m) = moles of solute/kg of solvent
a) 0.0 m NaCl solution
b) 0.5 m NaCl solution
c) 1.0 m NaCl solution
d) 1.5 m NaCl solution
e) 2.0 m NaCl solution
2. Next, you’ll set up your graph using the dataset. Click on the Graph tab at the top of the screen to switch to graph view. Be sure that the Scatter/Box/Bar or Categorical Bubble icon is selected. Click the Show buttons beneath the variable names to show the independent variable on the X-axis and the dependent variable on the Y-axis of the graph. Next, click on the Show button under the Group column and select the Z axis (on the right side panel). This will show each group’s data with its own color. Finally, check the Regression Line box.
3. Determine the slope of the regression line from your graph. Don’t forget units!
Use the slope of the regression line and the equation: ΔTR=imKR to calculate the boiling point elevation constant (Kb) of water.
Show your calculations below and include units.
Keep the following in mind:
A) ΔTB is the change in the boiling point (°C)
B) i is the van’t Hoff factor
C) m is the molality of the solution
D)KB is the boiling point elevation constant of water
5. The accepted value for the boiling point elevation constant (KB) of water is 0.512 °C/m. Calculate the percent error based on the dataset. Include units.
6. Use the dataset and the graph to make a claim about the relationship between the concentration of the sodium chloride solution and its boiling point.
7. How would you expect the slope of the regression line in the graph to change if the aqueous solutions in this experiment were made with MgCl2 rather than NaCl? What if the solutions were made with sucrose rather than NaCl?