Diluting Aqueous Solutions (page 300-304)

Diluting Aqueous Solutions (page 300-304)

Lesson 3

December 12th and maybe part of the 13th.

Solutions can be prepared two ways; solids can be added to liquids or liquids can be added to liquids.

Standard Solution A solution with precisely known

concentrations that have been prepared in labs and industry.

Preparing standard solutions takes skill and precision equipment to measure the mass (electronic balance) and volume (volumetric flask and/or volumetric pipet).

Preparation of Standard Solution

Dilution is a common scientific procedure that begins with a stock or standard solution which is added to a solvent to decrease the concentration to the desired value.

The concentration is inversely related to the solution’s volume.

C1 =

Concentration of solution 1

v1 = Volume of

Solution 1

C2 =

Concentration of solution 2

v2 = Volume of

Solution 2

As long as three of the values are known, the forth can always be calculated.

Rules to rearranging equations RULE #1: you can add, subtract, multiply

and divide by anything, as long as you do the same thing to both sides of the equals sign. 

In an equation, the equals sign acts like the fulcrum of a balance: if you add 5 of something to one side of the balance, you have to add the same amount to the other side to keep the balance steady

Rule 1 example

Find b in the equation y = mx + b

Subract mx from each side of the equation

Y – mx = mx – mx + b Y – mx = b

RULE #2: to move or cancel a quantity or variable on one side of the equation, perform the "opposite" operation with it on both sides of the equation. 

For example if you had g-1=w and wanted to isolate g, add 1 to both sides (g-1+1 = w+1). Simplify (because (-1+1)=0) and end up with g = w+1. 

More complex example This equation is used to calculate the

density of icebergs

We will isolate ρobject which is the density of the iceberg itself.

Start by isolating the equation in brackets Divide both sides by Htotal

=

=

Next we will isolate the faction that contains ρobject

Subtract 1 on each side

=

Multiply both sides by ρfluid to get rid of the fraction

Cancel out ρfluid

We will now remove the negative sign by multiplying by -1

=

Rearrange the equation a little

And we are done

Example 1

How can we prepare of 100 mL of 0.040 mol /L K2Cr2O7 from 0.200 mol/L K2Cr2O7?

c1= 0.040 mol / L

v1 = 100 mL

c2 = 0.200 mol / L

v2 = ?

Example 1

= 20 ml

It asked for us to prepare 100ml. Volume of water = 100 mL – 20 mL = 80 ml Therefore, we would need 20 mL of 0.200 M

of K2Cr2O7 and 80mL of water to form 100 mL of 0.040 M K2Cr2O7.

Example 2

How can a student prepare 123mL of 1 % Normal Saline solution from a 3% saline solution?

v1 = 41 mL

C1 = 3 % NS

v2 = 41 mL + 82 mL = 123mL

C2=?

= 1 % Therefore, the student made a 1% NS

solution when they added 82 mL to 41 mL of 3 % NS.

Preparation of Standard Solution

Preparing a Standard Solution from a Solid Reagent

1. Calculate the required mass of the solute from the volume and concentration of the solution.

2. Obtain the required solute in a clean, dry beaker or weighing boat.

3. Dissolve the solid in pure water using less than one half of the final solution volume.

4. Transfer the solution and all of the water used to rinse the equipment into a clean volumetric flask. (All of the equipment should be rinsed two to three times.)

5. Add pure water, using a medicine dropper for the final few millimetres until the bottom of the meniscus reaches the calibration line.

6. Stopper the flask and mix the solution slowly by inverting the flask.

Preparing a Standard Solution by Dilution

1. Calculate the volume of the concentrated reagent required.

2. Add approximately one half of the final volume of pure water to the volumetric flask.

3. Measure the required volume of stock solution using a pipet.

Pipet technique

Rinse the pipet with small volumes of distilled water.

A clean pipet has no visible residue or liquid drops clinging to the inside wall.

Attach the bulb to the pipet and place the pipet around 1 cm from the bottom of the container. Make sure that it does not touch the sides of the container.

Draw the solution into the pipet. If using a bulb pipet that must be removed, draw the liquid past the calibration line.

Wipe all solution from the outside of the pipet using a paper towel.

While holding the piper vertically, touch the pipet tip to the inside wall of a clean receiving container. Release the contents into the container.

Depending on the type of pipet, the flow of fluid may need to be stopped at a calibration line at the bottom.

Finish by holding the pipet tip at a 45 degree angle. Do not shake the pipet as most are calibrated to have a small volume in the tip.

Preparing a Standard Solution by Dilution

4. Transfer the stock solution slowly into the volumetric flask while mixing.

5. Add pure water and then use a medicine dropper for the final few millilitres until the bottom of the meniscus rests on the calibration line.

6. Stopper the flask and mix the solution by slowly inverting the flask several times.

Safety Tip

When carrying out a dilution with an acid.  Always add acid to water. 

Never add water to the acid.   When acid and water mix a great deal of

heat is released. Acid added to water gives a large heat sink for the heat to dissipate in.  If you add water to acid, the heat generated will cause the water to boil, and may splatter you with the hot acidic solution. 

Home work + Practice

Page 306 # 6 – 8, P 306 #3-5, 8ab, 9.

Practice using the tools with proper technique.

Lab Tomorrow Read 360 - 374