Concentration and

Dilution of Solutions

HOW TO MAKE A SOLUTION

“The Procedure”

We’ll make a Kool-Aid Solution!!!

Problem: How would you prepare 100 mL of a 2.5 g/L solution of Kool-aid?

#1. CALCULATE HOW MUCH SOLUTE YOU NEED ( Translation: How much SOLID Kool-aid is needed? )

Concentration = g of solute/L of solution

#2: Mass the amount of solute that YOU calculated on a balance.

#3: Take an appropriately sized volumetric flask. Fill roughly half the volumetric flask with water.

(if you are making a 100 mL solution, the volumetric flask must be 100 mL)

0.25 g of solute

#4. Using the weighing boat, carefully add the ___ g of solute into volumetric flask.

#5. SWIRL the volumetric flask until all the solute has properly dissolved.

#6. Add water until the calibration line. Make sure the bottom of the meniscus is on the line!!!!

7. Cap and INVERT (carefully!) the volumetric flask. (This is to make sure that the solution is mixed).

In Class Example #1How would you prepare 250 mL of a 5g/L NaCl

solution?

In Class Example #2How would you prepare 250 mL of a 5%(m/v)

NaCl solution?

Arrange the following four solutions in increasing concentration?

1) 2.5 g/L 2) 3.0 g/L3) 1.3 g/L

4) 1.45 g/L

PROCEDURE ON HOW TO DILUTE A SOLUTION

A dilution of a solution is a less-concentrated form of that solution.We dilute a solution by adding more solvent.

How would you prepare 200 mL of a 1.0 g/L solution of Kool-aid from the original 2.5 g/L

solution?

You MADE this solution!

Concentration: 2.5 g/L

You have 100 mL

You WANT this solution!

Concentration: 1.0 g/L

You WANT 200 mL

1. Calculate how MUCH of your original solution is required (the Vc!).

CcVc = CdVd

Cc = Concentration of the CONCENTRATED solution

Vc = Volume of the CONCENTRATED solution

Cd = Concentration of the DILUTED solution

Vd = Volume of the DILUTED solution

Calculation

CcVc = CdVd

(2.5 g/L)* Vc = 1.0 g/L * 200 mL

Vc = 1.0 g/L * 200 mL (2.5 g/L)

Vc = 80 mL

200 mL

3. Pour the 80 mL of concentrated solution into the volumetric flask.

4, Add water until the 200 mL mark.

Vc = 80 mL

2. Take out 80 mL and put it in the graduated cylinder

5. INVERT.

Serial Dilutions

If we wanted to make a 0.001g/mL solution of salt water (or a 1:1000 solution) what would be an easy way to do so?

We could take 1g of salt, and add it to 1000mL (or 1L) of water(1g/1000mL = 0.001g/mL)

What is one problem with this setup?

We now have 1 L of our solution! This is a lot – what if we only need 10mL?

Serial dilutions allow you to use small amounts of various dilutions to reach the concentration you want.

Serial dilutions are usually made in increments of 10, 100 or 1000

Figure illustrating serial dilutions

Using our formula CcVc=CdVd we can start by making 10mL of a 1:10 dilution (can also be written as 0.1 or 10%):

C1 = 1 (full strength original)V1 = this is what we are trying to find Doing the math: C2 = 0.1 (10% or 1:10) 1V1 = 0.1 x 10V2 = 10mL V1 = 1mL

So, to get 10mL of a 1:10 solution, we must add 1mL of our stock solution to 9mL of water

Now that we have a 1:10 dilution, we can use it to make 10mL of a 1:100 dilution (0.01 or 1%)

C1 = 0.1 V1 = trying to find this Doing the math:C2 = 0.01 0.1V1 = 0.01 x 10V2 = 10mL V1 = 1mL

So, to get 10mL of a 1:100 dilution, we add 1mL of our 1:10 dilution to 9mL of water

Now that we have a 1:100 dilution, we can use it to make 10mL of a 1:1000 dilution (0.001 or 0.1%)

C1 = 0.01 V1 = trying to find this Doing the math:C2 = 0.001 0.1V1 = 0.01 x 10V2 = 10mL V1 = 1mL

So, we can add 1mL of our 1:100 dilution to 9mL of water to get 10mL of a 1:1000 dilution!

PROBLEMS1. How would you prepare 1L of saline(salt) solution with a

concentration of [10-1].

2. How would you prepare 1L of a [10-2] saline solution using your [10-1] stock solution from question 1.

3. You have 10 mL of a [10-2] solution. How would you prepare a 10 mL [10-3] solution using the solution available to you in the lab?

4. You have 1000 mL of a [10-1] stock salt solution.

a. How would you make 35 mL of a [10-2] salt solution using your stock solution? SHOW ALL YOUR WORK AND INDICATE A PROCEDURE.

 

b. How would you make 50 mL of a [10-3] salt solution from part a? SHOW ALL YOUR WORK AND INDICATE A PROCEDURE.