Recall

Definitions

• Solute – (n) the part of a solution present in the LEAST amount
(what is dissolved)
• Solvent – (n) the part of a solution present in the greatest amount

(what does the dissolving)

• Solution – (n) a homogeneous mixture containing a solute and a solvent mixed together

Introduction

Look at the two beakers (A and B) of pennies and macaroni as an analogy of a solution (the pennies are the solute and the macaroni are the solvent.)

1)      What type of mixture are these, really?

2)      Are they solutions (consisting of a solute and a solvent)?

3)      What is the ratio of pennies to macaroni (write as a fraction  # of pennies / # of macaroni) of each?

4)      If you reach in to beaker A and take out what you grab, are you likely to grab a penny? What about beaker B?

5)      Which beaker has a greater “concentration” of pennies?

6)      Which beaker has a more “dilute” “concentration” of pennies?

Definitions

• Concentration – (n) the measurement of how much solute there is in a solution (or generally – the amount of substance found in a certain area or volume)
• Dilute – (adj) a solution is said to be dilute if it has less solute.
• Concentrated – (adj) a solution is said to be concentrated if it has more solute.

Exploration

In front of you there is a beaker of macaroni and pennies with some extra pennies and macaronis in a bag.

1)      Find two ways to concentrate (increase the concentration) the pennies in the beaker.

2)      Find two ways to dilute (decrease the concentration) the pennies in the beaker.

Definitions

• Diluting– (v) the process of adding more solvent to a solution or removing some of the solute
• Concentrating – (v) The process of adding more solute or removing some of the solvent.

Calculation

In Chemistry the concentration of a solution is most often reported in Molar (M) units or moles of solute per liter of solvent (mol / L).

For example:  We dissolve 58.44 g of NaCl (salt) in a liter (L) of water in a bottle.

58.44 g NaCl    x    1 mol NaCl     =     1 mol NaCl      or   a 1.000 M NaCl solution
1 L of H2O             58.44 g NaCl             1 L H2O

We have created a 1.000 Molar solution of NaCl or in other words a solution that contains 1 mole of NaCl per liter of water.

In front of you there is a flask that has a solution of 0.250 L of water with 14.61 g of NaCl dissolved in it.

1) What is the molar concentration of the solution?

Application

Chemists have special containers called volumetric flasks for making solutions.  The flasks have one precisely measured volume mark on a skinny neck for accurate measurements and a large bottom so that the solute can be swirled to dissolve.

Here are the steps Chemists take to make a solution:

A.     Add your solvent (often water) until 2/3 of the large bottom is filled with water.

B.     Accurately mass the appropriate amount of solute.

C.     Carefully transfer ALL of the solute into the flask.

D.     Swirl flask until all of the solute is dissolved (Be Careful not to spill any of your solution or it will be inaccurate)

E.      Carefully add solvent, swirling occasionally, until the meniscus of the solution reaches the volume line.  If you go over you must start all over again.

1)      If one wanted to make a 2.00 molar solution of baking soda (NaHCO3) in a 0.500 L volumetric flask, how many grams of baking soda would be needed?

2)      Make this solution using the materials available to you.

*Synthesis (Critical Thinking)

Some salts when dissolved in water make the level of the solvent decrease when they are added because of the way the particles interact with each other in solution.  You are making up a 1 molar solution of one of these salts.

If you follow the procedure above, will your solution be more concentrated than, more dilute than, or exactly 1 molar?  Explain why?

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