Concentrations+of+solutions

Objectives:
1. Solve problems involving the molarity of a solution.
 * Molarity** is simply a measure of the "strength" of a solution.

number of moles of solute

Liters of solution Example 1. What is the molarity of a 5.00 liter solution that was made with 10.0 moles of KBr ?

Solution: We can use the original formula. Note that in this particular example, where the number of moles of solute is given, the identity of the solute (KBr) has nothing to do with solving the problem. number of moles of solute

Liters of solution Given: # of moles of solute = 10.0 moles Liters of solution = 5.00 liters 10.0 moles of KBr -- = 2.00 M 5.00 Liters of solution
 * Answer = 2.00 M**

Example 2. A **250 ml (0.25 L)** solution is made with 0.50 moles of NaCl. What is the Molarity of the solution?

Given: Number of moles of solute = 0.50 moles of NaCl Liters of solution = 0.25 L of solution 0.50 moles of NaCl --- = 2.0 M solution 0.25 L Example 3. What would be the volume of a 2.00 M solution made with 6.00 moles of LiF?
 * Now, solve the problem as you solved example 1.**
 * Answer = 2.0 M solution of NaCl**

Solution: Given: # of moles of solute = 6.00 moles Molarity = 2.00 M (moles/L)

Liters of solution = 6.00 moles 2.00 moles/L
 * Answer = 3.00 L of solution**

2. Describe how to prepare dilute solutions from more concentrated solutions of known molarity. **__WITH MATH:__
 * __ Describe how to prepare dilute solutions from more concentrated solutions of known molarity __**
 * __ WITHOUT MATH __** : If CONCENTRATION is the amount of solute dissolved in a given amount of solvent, then to decrease the amount of solute per amount of solvent, you add more solvent while keeping the solute the same.

Use the C1V1 = C2V2 equation __Key:__** C1 = The initial concentration of your stock solution V1 = The required volume of your stock solution This is your unknown (?) C2 = The initial concentration of your solution you are preparing V2 = The final concentration of your solution that you are preparing

We need 100 ml of 10 mM Tris pH 7.6. We have a soltuion of if 1 M Tris pH 7.6. C1 = 1 M C2 = 10 mM V1 = ????? V2 = 100 ml V1 = C2V2 / C1 V1 = (10 mM) (100 ml) / 1 M

V1 = (10 mM) (100 ml) / ( 1 M ) x (1M)/(1000mM) = 1 ml Take 1 ml of 1 M Tris pH 7.6 to a total volume of 100 ml of solution.

Practice Problems: 1. You want to make 20.0 mL of 0.10 M NaCl solution. You have a 2.00 M stock solution. a. How many moles of salt do you need? b. Using your answer from part a, what volume of the concentrated solution do you need? c. Use the dilution equation to calculate the volume you need.

2.You take 250 mL of 6.00 M nitric acid, and add water until the final volume is 2.00 L. a. How many moles of nitric acid are in the sample you take? b. Using your answer from part a, what is the concentration of the nitric acid in the final solution? c. Use the dilution equation to calculate the final concentration.

Answers: 1. a) .0020 mol b) 1 mL (.0010 L) c) answers will vary

2. a) 1.5 mol b) .750 M c) answers will vary 3. Explain what is meant by percent by volume and percent by mass. Both represent concentration, percent by volume is usually for liquids and percent by mass is for solids. volume of solute --- X 100 = Percent by volume solution volume

Mass of solute (g) -- X100 = Percent by mass Solution volume (mL)

4. Be able to properly use terminology such as: concentration, dilute solution, concentrated solution, molarity. Addresses Objective 4 Concentration- The measure of the amount of solute that is dissolved in a given quantity of solvent Dilute Solution- Solution that contains only a low concentration of solute Concentrated Solution- Has a HIGH concentration of solute Molarity- The number of moles of a solute, dissolved per liter of solution (also called molar concentration) Describe how to prepare dilute solutions from more concentrated solutions of known molarity
 * __The Basics- You must know these terms…there is no way around it __**

WITHOUT MATH:

If CONCENTRATION is the amount of solute dissolved in a given amount of solvent, then to decrease the amount of solute per amount of solvent, you add more solvent while keeping the solute the same.

Isn’t that easy?

WITH MATH:

Ok…so here goes nothing.

First, write the definition of Molarity as a mathematic equation:

Molarity = moles of solute÷ liters of solution

Now: plug in your numbers!!!

Your answer will be written as: some number M

Not so bad….

Sample Problems!!!! HOOORAYYYY!!!

Q: A saline solution contains .8 g of CO2 in exactly 120 mL of solution. What is the molarity of the solution????

OK here is what we need to do:

1) look at equation:

M=moles of solute/ liters of solution

WE HAVE A PROBLEM!!! WE KNOW WE HAVE 120 mL OF SOLUTION…THAT’S NOT LITERS!!!!

2) convert the mL to liters

1000 mL in a liter…..

sooo… 120…move the decimal three spaces….to the…ummm….left

120 mL = .120L

3) PLUG IT INTO THE EQUATION!!!

WAIT…dang….we don’t know the molar mass….ok we’ll do that

C=12 O=16 (16*2)+12=44

44g per mole….. and .08g

.08/ 44=number of moles=0.0018 moles WOW…what a lame number

4) ok…so NOW plug it in

M= 0.0018/ 0.120L = 0.015

5) Answer: 0.015 M… ok…only like, 5 more to go

Try these on your own:

1) A solution has a volume of 4 L and contains 27 g of hydrogen. What is the molarity of the solution? 2) A solution has a molarity of 34. It contains 1500 mL of solution. How many moles of CN (cyanide) is dissolved in the solution??

WAIT…DON’T LOOK BELOW!!!! IT’S THE ANSWERS!!!!

1) 6.75 M 2) 51 moles of cyanide

Links: http://www.ausetute.com.au/concsols.html DON’T USE THE AUTOMATIC CALCULATOR THING THEY HAVE HERE!!!! Just check out their descriptions of different concepts and further eqns, as well as links to tests