5.1.7: Solutions- from scratch and by dilution

Lab #7 CHEM 0103 Chemistry of the life sciences (follow-up experiment: Lab #8, in a week)

Solutions: Intravenous fluids

Two of the most common intravenous fluids given to patients are saline (to replace fluids for dehydrated patients) and glucose (to provide energy for a patient that can't eat ). These fluids have to match blood plasma in the number of dissolved ions/molecules to prevent osmotic shock (we will learn about osmosis later, but basically red blood cells will shrivel or pop if there are too many or too few ions/molecules dissolved in fluids given intravenously).

Today, we will make solutions of NaCl (or glucose, depending on which lab group you are in) of a given concentration, and later study three of their properties (density, conductivity, refractive index). We will learn two methods of making solutions, a) from scratch and b) by diluting a solution of known concentration.

The concentration of a solute dissolved in a solvent is given by the chemical amount of solute divided by the total volume of the solution. For instance, a salt solution with a concentration 4 mol/L would contain 4 moles of NaCl in a final volume of one liter. A glucose solution of 0.5 mol/L would contain 0.5 mol in a final volume of one liter. Making a solution always involves five steps.

1. Transfer the necessary amount of solute into a beaker.
2. Add water and mix, making sure the volume is below the desired final volume. For high concentration glucose solutions, switch step 1 and 2 to prevent glucose from caking at the bottom of the beaker.
3. Transfer the solution into a volumetric flask or graduated cylinder, and fill up with water to the desired volume.
4. Mix the finished solution.
5. Transfer it to a labeled container for storage.

We don't have an instrument to measure chemical amount of solutes directly. If the solution is made from scratch, we use a balance to weigh out the solute. The mass m and the chemical amount n of solute are related by the molar mass M: m = n * M. For example, the molar mass of NaCl is 58.55 g/mol. If you need 4 moles of NaCl, you would weigh out a mass of m = n * M = 4 mol * 58.55 g/mol = 234.2 g. After you weighed out the correct amount, you have to make sure all of it gets into your beaker. You may use water to wash any solute remaining on the weighing dish into the beaker.

If the solution is made by diluting an available solution of known concentration, you obtain a given chemical amount of solute by transferring a defined volume of that so-called stock solution. If we know the concentration of the stock solution c_stock, we can calculate what volume of stock solution we need to use to get the chemical amount n: \(V_\mathrm{stock} = n/ c_\mathrm{stock}\). The concentration of the diluted solution depends on the volume of the final solution (which depends how much water you add): \(c_\mathrm{diluted} = n/ V_\mathrm{diluted}\). You can combine the two relationships to arrive at the dilution law, \(c_\mathrm{diluted} \cdot V_\mathrm{diluted} = c_\mathrm{stock} \cdot V_\mathrm{stock}\).

Task 1

Write out an experimental protocol to make 50 ml of an aqueous solution of from pure solid solute and pure water. The type of solute (NaCl or glucose) and its concentration will be different for each group. Discuss how your protocol will be different from those of other groups, and compare your protocol with theirs to check your work. Then, write out an experimental protocol to make a solution with a concentration 20 times lower than your first solution. Discuss how the dilution protocol will be different from those of other groups, and compare your protocol with theirs to check your work.

Task 2

In the five step procedure of making a solution, you have to pay attention to chemical amount of solute in step one, volume of the solution in step three, and concentration of your solution in step five. Based on this, answer the following questions. For each part, you might decide that there is no problem and you can proceed, or there is a problem you can fix, or you have to start over again.

1. What do you do if in step one, you took

• too much solute
• too little solute

2. What do you do if in step two, your volume is

• larger than the desired volume
• smaller than the desired volume

3. What do you do if in step three, some of the solution

• remains in your beaker
• spills on the bench

4. What do you do if in step four, your volume is

• larger than the desired volume
• smaller than the desired volume

5. What do you do if in step five, some of the solution

• remains in your volumetric flask or graduated cylinder
• spills on the bench

Task 3

Make the two solutions assigned to your group and transfer them into labeled 50 mL tubes. The label should include a date, your group name, the concentration and the name of the solute. Everyone will be using these for the next lab, so please label them carefully.

• Your group name and weekday you meet, e.g. group Carbon Monday
• The date (important for eventual disposal)
• What the solute is (e.g. NaCl or glucose)
• What the concentration of your solution is (e.g. 1.2 mol/L)

Please give your solutions to the instructor at the end of lab for safekeeping (we will use them again in two weeks).