Module 6 lab

Say you have 500 mL of a solution (HCl and water, for example) in a volumetric flask, and the concentration of HCl is 5 mol/L.

If you want to dilute this solution to 2 mol/L, first you use the equation:

Concentrationinitial * Volumeinitial = Concentrationfinal * Volumefinal Or shortly put: CiVi = CfVf

Since our final concentration will be 2 mol/L, we know that Cf is 2. Ci is 5 (don't worry about the units) and Vi is 500.

(5)(500) = (2)(Vf)

Vf = 2500/2 = 1250 mL

What does this number mean?

In order to dilute the 5 mol/L into 2 mol/L, we need 1250 mL of solution. We already have 500 mL, so we'll need 750 mL of water added onto the 500 mL (750 mL = 1250 mL - 500 mL). This will give us a diluted solution of HCl to the concentration we want.

• Here's another method or way of thinking about it...

If we want to prepare 500.0 mL of a 0.0540 mol/L NaOH solution, begin by thinking what the number means. That is, in one L there are 0.0540 moles of NaOH. NaOH has a molar mass of 40.01 g/mol, so 0.0540 moles is a very small mass.

So we need to begin by calculating how much NaOH is required:

Step 1: Convert 500.0 mL to litres.  (= 0.500 L)

Step 2: Convert litres to moles using molarity as the conversion factor. 
(0.500 L x 0.0540 mol/L = 0.027 mol)

Step 3: Convert moles of NaOH to mass of NaOH (in grams) using molar mass as the conversion factor.


Thus, we need to weigh out 1.08 grams of NaOH on an electronic balance, add it to a 500 mL volumetric flask, add approximately 300 mL of distilled water, swirl to dissolve the solid NaOH and add more distilled water until the 500 mL mark is reached on the volumetric flask (see diagram below).

Here's a bit of a tutorial.

<a rel="nofollow" href="https://http://highered.mcgraw-hill.com/sites/dl/free/0072857684/322633/10_prep_solu_dilution.swf" target="_blank">https://http://highered.mcgraw-hill.com/sites/dl/free/0072857684/322633/10_prep_solu_dilution.swf</a>

Thanks Chemb. Unfortunately I am still totally lost. I have no clue on how to start this report and how to fit the answers to these questions in it.
This is what I have so far.

Module 6 Lab: Dilutions and Recrystallization


PART I--Dilutions

 Prepare a report that describes, in detail, the technique of diluting a concentrated solution to a more dilute concentration using a volumetric flask.

In your report, be sure to include:

(a) the technique of transferring an exact amount of liquid from one container to another. Pipetting involves drawing a liquid into a pipet and allowing liquid to drain from the pipet in a controlled manner. Pipetting is used to quantitatively transfer exact volumes of a liquid from one container to another.
(b) the process of achieving a precise volume in one container while still being able to thoroughly mix the contents. The process of achieving a precise volume in one container


Be sure to describe the entire dilution process--the starting process, the addition of water, the role of the meniscus, the mixing process, stoppering the flask in final process, etc
When performing a dilution the starting volume of a liquid must first be transferred to a volumetrick flask using a pipette. The water should be added a little at a time to allow the solution to be properly mixed. When the meniscus of the water is near the fill line, a disposable pipette can be used to bring the level of the water precisely to the fill line. Once the bottom of the meniscus is even with the fill line, stopper the flask and invert mix.

Finally, give a specific example and explain how you use dilution in your everyday life.


PART II--Recrystallization

After watching the video, prepare a report that describes the use of recrystallization as a method of purifying a solid. How does it work? What do you do?

In your report, be sure to discuss:

(a) the process of making a supersaturated solution of acetyl salicylic acid.
(b) the process of recrystallization.
(c) the role of vacuum filtration in this process.

At the end of your report, please define the following terms:

(a) Saturated solution-A solution in which the maximum amount of solvent has been dissolved. Any more solute added will sit as crystals on the bottom of the container.
(b) saturation point- The point at which a substance will receive no more of another substance in solution, chemical combination, etc.
(c) supersaturated solution- A solution that contains a higher than saturation concentration of solute; slight disturbance or seeding causes crystallization of excess solute

Hey again,

I'm thinking your making this harder than it actually is I'm assuming they want two paragraphs per report.

First talk about what a dilution is:

PART I - it practically writes itself

A dilute solution is a solution containing a relatively small quantity of solute as compared with the amount of solvent.  A concentrated solution is a solution containing a relatively large quantity of solute as compared with the amount of solvent. When diluting solutions, we typically use volumetric flasks, as they are typically used for making up solutions to a known volume. In order to be exact when transferring liquid from one flask to another, we use a graduated cylinder; this measures a precise about of liquid due to calibrated size. However, when only a small about is being transferred, a pipette is used. Pipetting involves drawing a liquid into a pipet and allowing liquid to drain from the pipet in a controlled manner. Pipetting is used to quantitatively transfer exact volumes of a liquid from one container to another. In order to a precise volume in one container, calculations are make prior; the quantitative values depends on how much of the original solution is being diluted. Shown below is a sample of how calculation would be performed:

Say you have 500 mL of a solution (HCl and water, for example) in a volumetric flask, and the concentration of HCl is 5 mol/L.

If you want to dilute this solution to 2 mol/L, first you use the equation:

Concentrationinitial * Volumeinitial = Concentrationfinal * Volumefinal Or shortly put: CiVi = CfVf

Since our final concentration will be 2 mol/L, we know that Cf is 2. Ci is 5 (don't worry about the units) and Vi is 500.

(5)(500) = (2)(Vf)

Vf = 2500/2 = 1250 mL

What does this number mean?

In order to dilute the 5 mol/L into 2 mol/L, we need 1250 mL of solution. We already have 500 mL, so we'll need 750 mL of water added onto the 500 mL (750 mL = 1250 mL - 500 mL). This will give us a diluted solution of HCl to the concentration we want.

Generally, when performing a dilution the starting volume of a liquid must first be transferred to a volumetrick flask using a pipette. The water should be added a little at a time to allow the solution to be properly mixed. When the meniscus of the water is near the fill line, a disposable pipette can be used to bring the level of the water precisely to the fill line. Once the bottom of the meniscus is even with the fill line, stopper the flask and invert mix.

And for PART II, I wrote a whole post on it, did you read it? Or you still need help with it?

Don't worry, I'll be around.

Hey Chemb, knowing me your probably right, I am probably making it more harder than it actually is! Thanks so much for thoroughly walking me through this particular assignment. As for part 2 I did read your prior post and I think I can pretty much get through the second part. If I end up needing help with, your the first one Im looking for.
I guess Im freaking out about this particular assignment because it is my last one in Chem and I am holding an 80 right now, and I have to pass Chem if I want my forensics certificate. Thanks a bunch you have been extremely helpful!

Ya, just post back when you need me.