SCH 3U - Solution Prep

Preparing a Solution from a Solid

ex. What mass of sodium hydroxide is required to make 500.0 mL of 0.200 mol/L solution?

m = ?                            n = cV                                            m = nM

V = 0.5000L                    = 0.200 mol/L(0.5000L)               m = 0.100 mol(40.00 g/mol)

c = 0.200 mol/L               = 0.100 mol                                     = 4.00 g

M = 40.00 g/mol

                                    ∴ the mass is 4.00 g.

Procedure:

1. Complete calculations to determine mass of solid solute required.
2. Mass out the solid solute on the electronic scale.*
3. Add about 3 cm of distilled H₂O to an appropriately sized volumetric flask.**
4. Place the solid solute into the volumetric flask, using a powder funnel.
5. Dissolve the solid solute by swirling the flask.
6. Fill the flask to the line*** with distilled water and label.****
7. Top up to line when solution reaches room temperature.*****

 Safety:  goggles; gloves and apron when appropriate

Tips and Tricks:

 * If using a hygroscopic solid (a substance that picks up moisture from the air), like sodium hydroxide, measure out the substance as quickly as possible and place the lid back on the stock bottle asap. 

** A volumetric flask is used because it has a high degree of accuracy.  Other measuring equipment that measures volume accurately includes graduated cylinders, pipettes and burettes.  Equipment that measures volume poorly includes beakers and Erlenmeyer flasks.

 

 *** A volumetric flask has a line etched into the neck to show you where to fill to attain the desired volume.  You must take into consideration meniscus and parallax when filling. 

**** Label should include your name, as well as the chemical's name, concentration and the date it was prepared. 

***** Many solutions get hot during the dissolution process.  When the solution cools down, the solution level in the flask will drop so you must top it up with distilled water.

 

 

Here's a video showing the prep of a solution.  Extra added bonus:  the dude in the video has a posh accent - very classy.   (Note: typically, we will not dissolve the solute in a beaker first - we just go for the gusto and do all the dissolving directly in the volumetric flask).



Introducing a New Equation
Whether you place 1 mol of a salt into a teacup of water or into a bathtub of water you still have 1 mol of the compound.  Therefore, 

n_concentrated = n_dilute

and since n = cV

c_concentratedV_concentrated = c_diluteV_dilute

or simply written by replacing the letters with numbers,

c₁V₁ = c₂V₂

This equation will be useful in our next calculation.

Preparing a Solution from a Stock Solution

ex. What volume of stock hydrochloric acid (12.0 mol/L) is required to make 1.00 L of 0.150 mol/L solution?

V₁ = ?                                          c₁V₁ = c₂V₂

c₁ = 12.0 mol/L            12.0 mol/L(V₁) = 0.150 mol/L(1.00 L)

V₂ = 1.00 L                                     V₁ = 0.0125 L                         

c₂ = 0.150 mol/L                                 = 12.5 mL

                                        ∴ the volume is 12.5 mL.

We always convert to mL at the end because all our equipment measures in mL.

Procedure:

1. Complete the calculation to determine the volume of stock solution required.
2. Use a graduated cylinder to measure out the stock solution.
3. Add about 3 cm of dH₂O to the flask.
4. Place the stock solution into a volumetric flask (1.00 L), using a funnel.
5. Dissolve the stock solution.
6. Fill the flask to the line and label.
7. Top up to line when solution reaches room temperature.

 Safety:  goggles, gloves, apron, fume hood

 

 

Here's a video showing the preparation of an acid solution.

Homework:  

Practice Problems p. 386 # 53, 54, 57

Review Questions p. 390 # 3, 5

 

 


Student Questions:
Would you please post the solutions to pg 390 #3, 5?