Section 6.2—Concentration
description
Transcript of Section 6.2—Concentration
Section 6.2—Concentration
How do we indicate how much of the electrolytes are in the drink?
Concentrated versus Dilute
solute solvent
Lower concentration
Not as many solute (what’s being dissolved) particles
Higher concentration
More solute (what’s being dissolved) particles
Concentration
Concentration gives the ratio of amount dissolved to total amount
There are several ways to show concentration
Percent Weight/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
100)/%( solventmLsolutegramsVW
%(W/V) Example
Example:If you dissolve 12 g of sugar in 150 mL
of water, what percent
weight/volume is the solution?
100)/%( solventmLsolutegramsVW
%(W/V) Example
10015012)/%( mLgVW
Example:If you dissolve 12 g of sugar in 150 mL
of water, what percent
weight/volume is the solution?
100)/%( solventmLsolutegramsVW
8.0% (W/V)
%(W/V) Example #2
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)/%( solventmLsolutegramsVW
%(W/V) Example #2
100200?)/%(15 mLgVW
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)/%( solventmLsolutegramsVW
30 g of sugar
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
Quick Mole Review (Remember the mole road map ?!?)1 mole = 6.02 × 1023 moleculesThe molecular mass of a molecule is
found by adding up all the atomic masses in the atom
Molecular mass in grams = 1 mole of that molecule
Quick Mole Example
Example:How many moles are in 25.5 g NaCl?
Quick Mole Example
25.5 g NaCl = _______ mole NaClg NaCl
mole NaCl158.44
0.44
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
Example:How many moles are in 25.5 g NaCl?
Molarity
Molarity (M) is a concentration unit that uses moles of the solute instead of the mass of the solute
Molarity Simulation
solventLsolutemolesM
Molarity ExampleExample:
If you dissolve 12 g of NaCl in 150 mL of water, what is
the molarity?
Molarity Example
LmolesM150.021.0
Example:If you dissolve 12 g of NaCl in 150 mL of water, what is
the molarity?
solventLsolutemolesM 1.4 M NaCl
12 g NaCl = _______ mole NaClg NaCl
mole NaCl158.44
0.21
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! 150 mL of water = 0.150 L
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
LmolesM250.030.0
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 NaCl158.44
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 mLgVW ? = 17.5 g NaCl
Let’s Practice #2Example:
What is the %(W/V) of a 500. mL
sample of a 0.25 M CaCl2 solution?
Let’s Practice #2
LmolesM500.0
?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 CaCl2mole CaCl2
g CaCl2110.981
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
1005009.13)/%( mLgVW
? = 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
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 ionsIf 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 #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