Concentration and Ionic Solutionsmrhillsciences.weebly.com/uploads/3/7/5/0/37505843/... ·...

1. The “Concentration”

[ ] of a solution is a

measure of how

much solute is

dissolved in the

solvent and can be

measured in units

like ppm, g/L , %.= Solution!!

2. The unit of concentration most often used in

chemistry is Molarity: mol/L.(M)

Molarity is defined as:

So a 1M solution of NaCl contains 1 mole

of NaCl in 1L of solution.

How would you make 1 M NaCl…

1 mol of NaCl = 58.44g

Therefore, add 58.44 g of NaCl to a

container, and add H2O to make up 1L of

solution. = 1 M NaCl

solution

solute

V

molsM

Molarity = Mol

L

M = 2.5mol

1.5L

M = 1.67 mol/L

You would call this a 1.67 “Molar” solution… Or 1.67M solution!

Molarity = Mol/L

We have to get grams to mols, and ml to L…

𝟓. 𝟏𝟐𝒈𝑵𝒂𝑪𝒍 ×𝟏𝒎𝒐𝒍

𝟓𝟖.𝟓𝒈= 0.0875mol NaCl

𝟐𝟓𝟎. 𝟎𝒎𝒍 ×𝟏𝑳

𝟏𝟎𝟎𝟎𝒎𝒍= 0.2500L

M=0.0875mol

0.250L

= 0.350 M NaCl

Eg.3 What mass of NaCl would make up

2.0L of a 0.25M(mol/L) NaCl solution?

1. Find the moles of NaCl needed.

2. Change moles to mass.

3. Answer: Add 29.22 g of NaCl to a container then add enough water to make up 2.0L of solution.

mol5.0

gNaClmol

gNaClmol 22.29

1

44.585.0

L

molL

1

25.00.2

Eg. 4. What mass of NaOH is

contained in 3.50L of 0.200 M NaOH?

1. Find the moles of NaCl needed.

2. Change moles to mass.

3. Answer: 28 g of NaOH is present in the solution

mol70.0

gNaOHmol

gNaOHmol 28

1

0.407.0

L

molL

1

200.05.3

Molarity = mol/L

So we need to get the g to moles, and the ml to L

𝟏. 𝟖𝟗𝟑𝒈 ×𝟏𝒎𝒐𝒍

𝟗𝟖. 𝟏𝒈= 𝟎. 𝟎𝟏𝟗𝟑𝒎𝒐𝒍

𝟏𝒎𝒍 ×𝟏𝑳

𝟏𝟎𝟎𝟎𝒎𝒍= 𝟎. 𝟎𝟎𝟏𝑳

Molarity = 0.0193mol/0.01L

= 18.7M H2SO4

Hebden: Pg 98 #59a,e,

#60a,e, #61, #62, #63, #66,

#68, #69

Does the amount of the

solute change?

Do the mols of the solute

change?

What do you think

happens to the Molarity?

1. When H2O is ADDED or REMOVED (evaporated)

from a solution OR if two solutions are mixed

together:

1. The concentration or Molarity (M) changes

2. The moles/grams of solute does NOT change

So if moles don’t change… then…

Moles of initial solution = Moles of final solution

And we know that…

Mol x L = Mol

L

Molarity x Volume = Mole

1. The new concentration of the solution can be found using the formula:

where [ ]= concentration or molarity (M)

Mi = Molarity of initial

Vi= Volume of initial

Mf = Molarity of final(diluted)

Vf= Volume of final(diluted)

ffii VMVM

MM

L

LM

L

mol

LLMLM

VMVM

f

f

f

ffii

0.1

0.3

)0.3(

0.3

0.3

)0.20.1()0.1)(0.3(

(Q#1) A student has 500.mL of a 0.4M NaCl

solution. How much water is needed to make

this solution 0.10M?

LV

VMmol

VMLM

VMVM

f

f

f

ffii

0.2

)10.0(20.0

)10.0()500.0)(40.0(

(Q#2) A chemist adds water to 120 mL of a

6.0M solution of NaOH until the final volume is

2.0L. What is the molarity (M) of the resulting

solution?

MM

LMmol

LMLM

VMVM

f

f

f

ffii

36.0

)0.2(720.0

)0.2()120.0)(0.6(

(Q#3) What volume of 12M HCl acid must be

added to water to make 5.0L of 0.10M HCl

solution?

LV

molVM

LMVM

VMVM

i

i

i

ffii

042.0

50.0)12(

)0.5)(10.0()12(

(Q#4)What concentration results when 150. mL

of a 0.36M solution of magnesium sulphate

are added to 750. mL of water?

MM

LMmol

LLMLM

VMVM

f

f

f

ffii

060.0

)900.0(054.0

)150.0750.0()150.0)(36.0(

Pg. 102 Exercises

(78,80,82,83,85,87,89)

1. It is important to be able to find the

concentration of ions in a solution once the

ionic compound has dissociated because…

1. It is the ions in the solution that are

responsible for any conductivity or

chemical reactions that

may occur.

2. Often the [ions] will

determine whether a

reaction will occur or not.

Polyatomic ions (e.g , )

etc… remain intact during dissociation.

2

4SO3

4PO

3NO

)()()( aqaqs ClNaNaCl

)(2

4)()(42 2 aqaqsSONaSONa

)(3

4)(2

)(243 23)( aqaqsPOMgPOMg

Calculating ion concentrations!!e.g. What is the concentration of all ions in a 1.0 M

NaCl solution?

(1) Write out the dissociation equation for the ionic

compound.

(2) Write in the concentration before dissociation

occurs.

(3) Write in the concentration after dissociation

occurs.

ClNaNaCl

ClNaNaCl

1.0M 0.0M 0.0M

ClNaNaCl1.0M 1.0M0.0M

ClNiNiCl 33

3

Before: 2.0M 0.0M 0.0M

After: 0.0M 2.0M 3(2.0)M = 6.0M

2. The final molarity(M) of the ions is in the same ratio as the moles in the dissociation equation.

E.g. Find the ion concentrations in a 2.0M NiCl3 solution.

1. Sometimes two ionic

solutions are mixed

together that have a

common ion

present.

2. Until you learn

differently in

Chemistry 12,

assume that no

reaction occurs

when two solutions

are mixed.

1. Dilution method formula (if needed)

2. Dissociation equation

3. Calculation [ions] in solution

solution

solute

V

nM

ffii VMVM

ClMgMgCl 22

2

1) Don’t need a dilution equation as we are not

adding/diluting it

2) Write out the dissociation equation for each ionic

compound.

3) Find the concentration of each ion after dissociation.

AlCl3 Al 3+ + 3Cl-

Before: 0.25M 0.0M 0.0M

After: 0.0M 0.025M 3(0.025M) = 0.75M

Cl Ions

(1) Find the new concentration of each after

mixing using the dilution formula.

M

LDivideby

LM

LMLM

VMVM

M NaCl0.1

0.2

)0.2(0.2

)0.2)(()0.1)(0.2(

2211

(1) Find the new concentration of each after

mixing using the dilution formula.

M

LDivideby

LM

LMLM

VMVM

M NiCl0.2

0.2

)0.2(0.4

)0.2)(()0.1)(0.4(

3

2211

2. Write out the dissociation equation for each ionic compound.

3. Find the concentration of each ion after dissociation.

ClNaNaCl ClNiNiCl 33

3

Before: 1.0M 0.0M 0.0M

After: 0.0M 1.0M 1.0M

2.0M 0.0M 0.0M

0.0M 2.0M 6.0M

(4) Look for ions common to both equations

in order to find the total ion concentration by

adding them together.

MNa 0.1][

M

MMCl

0.7

)0.60.1(][

MNi 0.2][ 3