# The molarity or molar concentration

## The molarity or molar concentration

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# The molarity or molar concentration

MOLARITY

The molarity (or molar concentration) of a solution is the number of moles of solute dissolved in 1 L of solution.  It is expressed in moles per litre (mol/L, or M)

Molarity    =      amount of solute (in mol)

volume of solution (in L)

or:            C  =  n

V

Calculating Molarity – Example 1

Calculate the molarity of a solution that consists of 12.3 g of potassium nitrate dissolved in 50 ml of water.

Solution

First, you need to find the number of moles of potassium nitrate (KNO3).

M (KNO3)   =      39.10 + 14.01 + 3(16.00)

=      101.11 g/mol

n      =      12.3 g

101.11 g/mol

=      0.1216 mol

Divide the number of moles by the number of litres.

50 ml/1000 = 0.050 L

C      =      0.1216 mol

0.05 L

=      2.43 mol/L

The molarity of the solution is 2.43 mol/L.

Example 2

Calculate the mass of NaCl required to prepare 250 ml of a 0.850 mol/L solution.

Solution

First you need to calculate the number of moles of NaCl, using the concentration and volume.

C = n/V     à     n      =      VxC

=      (0.250 L)(0.850 mol/L)

=      0.212 mol

M (NaCl)    =      22.99 + 35.45

=      58.44 g/mol

m     =      (0.212 mol)(58.44 g/mol)

=      12.4 g

p. 316  19-24

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### The molarity or molar concentration

Another Mole Concept: The Molar Concentration

Some Definitions you need to remember:

Solution:         homogeneous mixture of two or more substances.

Solvent:           the component of a solution that exists in greater quantity.

Solute:                    the component of the solution that exists in smaller quantity.

Concentration: amount of a substance present in a given volume of solution.

Concentrated:  has relatively large amount of substance present in a given volume of solution.

Dilute:             has relatively small amount of substance present in a given volume of solution.

Molar concentration or Molarity:       number of moles of a particular substance in one liter of solution.

Unit for Molarity:      “mole/L” or “M”

1.0 M  is read as “ 1.0 MOLAR”

Symbol for concentration:                 [ ]        e.g.  [NaCl] = 1.2 M

The Formula for concentration:

Molar Concentration (c) =    moles (n)         c = molar concentration

Volume (V)      n = number of moles

V = volume, in litres

Source : http://www.msdiehl.com/resources/molar+concentration+notes.doc

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### The molarity or molar concentration

Molarity & Dilutions Notes

Concentration:  Measure of how much solute is dissolved in a specific amount of solvent/solution.

• A concentrated solution contains larger amount of solute
• A dilute solution contains a smaller amount of solute

Molarity (M):  moles of solute / liters of solution

• Also know as molar concentration
• It’s unit is M (read/pronounced a molar)

Example:

1) What is the molar concentration of a liter solution with 0.5 mol of solute?

Molarity = moles of solute / liter of solution

= 0.5 mol solute / 1 L solution

= 0.5 M

• A 100.5 mL IV solution containing 5.10g glucose (C6H12O6).  What is the molarity of the solution?  (MM of glucose = 180.16 g/mol)

Molarity = moles of solute / liter of solution

Step 1:  convert grams of solute glucose to moles solute using MM of glucose

5.10 g glucose x 1 mole glucose  = 0.0283 mol glucose

180.16 g glucose

Step 2:  convert mL of solution to Liters of solution

(Remember week 1 notes:  here we’re going from milliliters to liters so we’re going to a higher prefix this means we’re moving the decimal to the left 3 decimal places)

100.5 mL = 0.1005 L

Step 3:  solve for molarity

M = moles of solute/ liters of solution

= 0.0283 mol glucose / 0.1005 L solution

= 0.282 M

How to prepare a molar solution:

Given the desired molarity and volume you can mix a solution by:

Refer to example from class.

Also see p. 484 of text

• The total # of moles of solute does not change during a dilution (only solvent is added)

Dilution Equation:

M1V1 = M2V2

M1 = the initial molarity of the concentrated solution

V1 = the initial volume of the concentrated solution

M2 = the final molarity of the dilute solution

V2 = the final molarity of the dilute solution

Example:

What volume of 2.00 M CaCl2 stock solution would you use to make 0.50 L of 0.300 M CaCl2?

M1V1 = M2V2

M1 = 2.00 M

V1 = ?

M2 = 0.300 M

V2 = 0.50 L

2.00 M x V1 = 0.300M x 0.50L

V1 = 0.075 L

Source : http://chemistry2008.pbworks.com/f/Molarity+notes.doc

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