section 6.2—concentration how do we indicate how much of the electrolytes are in the drink?...
TRANSCRIPT
Section 6.2—Concentration
How do we indicate how much of the electrolytes are in the drink?
Objectives:•Calculate concentrations in percent concentration and molarity•Calculate the concentration of electrolytes in solutions of ionic compounds
Concentrated versus Dilute
solute solvent
Lower concentration
Not as many solute particles
Higher concentrationMore solute particles
Concentration
Concentration gives the ratio of amount of solute dissolved to total amount of solute and solvent
There are several ways to show concentration
Percent Mass/Volume
This is a method of showing concentration that is not used as often in chemistry
However, it’s used often in the food and drink industryFor example, your diet drink can might say you
have less than 0.035 g of salt in 240 mL. That would give you a concentration of
0.035 g / 240 mL, which is 0.015% solution
€
%(M /V ) =grams solute
mL solvent×100
%(M/V) Example
Example:If you dissolve 12 g of sugar in 150 mL
of water, what percent
mass/volume is the solution?
€
%(M /V ) =grams solute
mL solvent×100
%(M/V) Example
€
%(M /V ) =12g
150mL×100
Example:If you dissolve 12 g of sugar in 150 mL
of water, what percent
weight/volume is the solution?
€
%(M /V ) =grams solute
L solvent×100
8.0% (M/V)
Practice Problems
Determine the % M/V of a carbohydrate in a sports drink if it contains 14 g in 240 mL; if it contains 20 g in 240 mL.
A certain sports drink contains 0.050 g sodium chloride in 240 mL. What is its concentration in % M/V? What if it had 0.100 g in 240 mL?
Concentration using # of molecules
When working with chemistry and molecules, it’s more convenient to have a concentration that represents the number of molecules of solute rather than the mass (since they all have different masses)
Remember, we use moles as a way of counting molecules in large numbers
Mole Review
The molar mass of a substance is found by adding up all the atomic masses in the substanceExample: The molar mass of NaCl is 58.5 g
The molar mass of a substance in grams represents 1 mole of that substance. 1 mole of NaCl has a mass of 58.5 g
Practice
Determine the molar mass of each of the following substances.Ca(OH)2
Mg
C2H3OH
Cu(NO3)2
Review: Calculating # of Moles
If you are given the mass of a substance, you use the molar mass to determine the number of moles.Example how many moles are in 35 g of Mg?35 g x 1 mole = 1.4 moles
24.3 g
Practice:
How many moles are in 25.5 g NaCl?
25.5 g NaCl = _______ mole NaClg NaCl
mole NaCl1
58.5
0.436
1 mole NaCl molecules = 58.5 g
NaCl
11
23.0 g/mole35.5 g/mole
= 23.0 g/mole= 35.5 g/mole+
58.5 g/mole
Example:How many moles are in 25.5 g NaCl?
More Practice
Determine the number of moles in each of the following:
1.68.0 g CaCl2
2.9.2 g CH4
3. 70.1 g Al(OH)3
Molarity
Molarity (M) is a concentration unit that uses moles of the solute instead of the mass of the solute
€
M =moles solute
L solution
Molarity ExampleExample:
If you dissolve 12 g of NaCl to make
150 mL of solution, what is the molarity?
Molarity Example
L
molesM
150.0
21.0
Example:If you dissolve 12 g
of NaCl to make 150 mL of solution,
what is the molarity?
€
M =moles solute
L solution= 1.4 M NaCl
12 g NaCl = _______ mole NaClg NaCl
mole NaCl1
58.5
0.21
1 mole NaCl molecules = 58.5 g
NaCl
11
23.0 g/mole35.5 g/mole
= 23.0 g/mole= 35.5 g/mole+
58.5 g/mole
Next, remember to change mL to L! Divide by 1000.150 mL of water = 0.150 L
Practice Problems
1. Determine the molarity of 2.5 L of solution made from 125 g of AgNO3.
2. What is the molarity of a solution produced when 145 g of NaCl is dissolved in enough water to make 2.75 L of solution?
3. What is the molarity of a solution in which 85.6 g of HCl are dissolved in enough water to make 385 mL of solution?
4. 8.77 g of KCl is dissolved in sufficient water to make 4.75 L of solution. Determine the molarity of the solution.
Converting between the two
If you know the %(W/V), you know the mass of the solute
You can convert that mass into moles using molecular mass
You can then use the moles solute to find molarity
Converting from % to M Example
Example:What molarity is a 250 mL sample of 7.0 %(W/V) NaCl?
Converting from % to M Example
L
molesM
250.0
30.0
Example:What molarity is a 250 mL sample of 7.0 %(W/V) NaCl?
1.2 M NaCl
17.5 g NaCl = _______ mole NaClg NaCl
mole NaCl1
58.44
0.30
1 mole NaCl molecules = 58.44 g
NaCl
11
22.99 g/mole35.45 g/mole
= 22.99 g/mole= 35.45 g/mole+
58.44 g/mole
Remember to change mL to L! 250 mL of water = 0.250 L
100250
?)/%(0.7
mL
gVW ? = 17.5 g NaCl
Concentration of Electrolytes
An electrolyte breaks up into ions when dissolved in water
You have to take into account how the compound breaks up to determine the concentration of the ions
CaCl2 Ca+2 + 2 Cl-1
For every 1 CaCl2 unit that dissolves, you will produce 1 Ca+2 ion and 2 Cl-1 ions
If the concentration of CaCl2 is 0.25 M, the concentration of Ca+2 is 0.25 M and Cl-1 is 0.50 M
Let’s Practice #1
Example:You want to make 200 mL of a 15% (W/V) solution of
sugar. What mass of sugar do you
need to add to the water?
100)/%( solventmL
solutegramsVW
Let’s Practice #1
100200
?)/%(15
mL
gVW
Example:You want to make 200 mL of a 15% (W/V) solution of
sugar. What mass of sugar do you
need to add to the water?
100)/%( solventmL
solutegramsVW
30 g of sugar
Let’s Practice #2
Example:What is the %(W/V)
of a 500. mL sample of a 0.25 M
CaCl2 solution?
Let’s Practice #2
L
molesM
500.0
?25.0
Example:What is the %(W/V)
of a 500. mL sample of a 0.25 M
CaCl2 solution?
2.8 %(W/V) CaCl2
0.125 moles CaCl2 = _______ g CaCl2
mole CaCl2
g CaCl2110.98
1
13.9
1 mole CaCl2 molecules = 110.98 g
CaCl
12
40.08 g/mole35.45 g/mole
= 40.08 g/mole= 70.90 g/mole+
110.98 g/mole
100500
9.13)/%(
mL
gVW
? = 0.125 moles CaCl2
Let’s Practice #3
Example:What are the
molarities of the ions made in a 0.75 M
solution of Ca(NO3)2
Let’s Practice #3
Example:What are the
molarities of the ions made in a 0.75 M
solution of Ca(NO3)2
Ca(NO3)2 Ca+2 + 2 NO3-1
For every 1 Ca(NO3)2, there will be 1 Ca+2 and 2 NO3-1 ions
Ca+2 = 0.75 MNO3
-1 = 1.5 M