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