Two methods for Preparation of a
desired volume of a Molar Solution

1) Preparation from a
solid solute.

2) Preparation by Dilution
of a Concentrated Stock
Solution.
Molarity Definition

Molarity (M) =
moles of solute
Liter of solution
1 M NaCl solution: 1 Mole (58.5 g) of NaCl
dissolved in a total aqueous solution volume
of 1 Liter.
58.5 g
Solution = Solute + Solvent
Important Note: Unless otherwise specified,
assume that the solvent for all solutions
discussed in this course is WATER.
What volume of water would be required to
prepare 1 liter of a 1 M NaCl solution?

A) Less than 1 liter

B) Exactly 1 liter

C) More than 1 liter
ANS: A – Less than 1 liter. Solution = Solute (NaCl) + Solvent (H2O). The
solute takes up space when it dissolves, thus less than 1 liter of water is
required.
Preparation of a desired volume of solution
from a solid solute.






Calculate moles of solute
needed.
Calculate grams of solute
needed.
Weigh solute with scale, and
transfer to volumetric flask
calibrated to prepare desired
volume.
Rinse container with water to
make sure that solute is
transferred
Dissolve solute in small volume of
water.
Fill up to mark with water.
Preparing Molar Solution From Solid
Give detailed directions for the preparation of 250. mL of a
0.100 M solution of CuCl2 from solid CuCl2.
Required Materials:
• CuCl2
• Scale, small beaker, spatula
• 250. mL volumetric flask
• Water (wash bottle)
Preparing Molar Solution From Solid
Give detailed directions for the preparation of 250.
mL of a 0.100 M solution of CuCl2 from solid CuCl2.
• Step 1: Calculate moles:
• How could you determine the # of moles
from the given information?
Calculating Moles – mathematical vs.
conceptual approach
Rearrange Molarity Equation
Proportion Method
Molarity = Moles Solute
• Scale up or down moles in 1 liter
by using a proportion:
Liters solution
•
Solve for moles by multiplying by
liters:
How many
in 0.25 L
0.25 L
0.100 M = x moles
0.250 L
x = 0.100 M x 0.250 L = 0.0250 moles
Moles = Molarity x Liters
•
x moles = 0.100 moles
.250 L
1.00 L
x = 0.100 M x 0.250 L = 0.0250 moles
Moles = Molarity x Liters
1 liter
Preparing Molar Solution From Solid
Give detailed directions for the preparation of 250.
mL of a 0.100 M solution of CuCl2 from solid CuCl2.
Molarity = moles solute
Liters of solution
• Step 1: Calculate moles:
•
moles = Molarity x volume in liters.
Converting mL to L: 250.mL (1.00 L)
= 0.250 L ;
(1000. mL)
0.250 L (0.100 moles CuCl2) = 0.0250 moles CuCl2
(1.00 L)
Preparing Molar Solution From Solid
Give detailed directions for the preparation of 250.
mL of a 0.100 M solution of CuCl2 from solid CuCl2.
Molarity = moles solute
Liters of solution
• Step 2: Convert moles to grams: 0.0250 moles CuCl2 → ? g
•
Cu: 63.5
Cl2: 2 x 35.5 = 71.0
143.5 g/mol
0.0250 moles CuCl2 (143. 5 g CuCl2) = 3.59 g CuCl2
(1 mole CuCl2)
Preparation of Solutions
Weigh 3.59 g CuCl2; transfer
to 250 mL volumetric flask
Dissolve CuCl2
in a small
volume of water
Add water to line
Link to Preparation of Molar
Solution Video
Dilution of Solutions
Dilution of Solutions
Preparation of a desired volume of a Molar Solution
from Dilution of a Concentrated Stock Solution


Calculate volume of stock solution needed using
Dilution Equation: MCVC = MDVD
Transfer (using pipette or graduated cylinder)
required volume of stock solution to volumetric size
calibrated to prepare desired volume.
 Dilute with water up to calibration mark.
Dilution of solutions

Since moles are constant before and after dilution,
we can use the following formula for calculations.
MCVC = MD VD
 MC
=
Molarity of Concentrated solution
VC = Volume of concentrated stock
 MD = Molarity of Dilute solution
 VD = Volume of Dilute of solution

 Note:
MC > MD
The Dilution formula
E.g. if we have 1 L of 3 M HCl, what is M if
we dilute acid to 6 L?
M1 = 3 mol/L, V1 = 1 L, V2 = 6 L
M1V1 = M2V2, M1V1/V2 = M2
M2 = (3 mol/L x 1 L) / (6 L) = 0.5 M
Why does the formula work?
Because we are equating mol to mol:
V1 = 1 L
M1 = 3 M
M1V1 = 3 mol
V2 = 6 L
M2 = 0.5 M
M2V2 = 3 mol
Dilution of Solutions

Describe how you would prepare 250. mL of a 0.100 M
CuCl2 solution, starting with a 0.500 M CuCl2 stock
solution.
Mc = 0.500M
VC = ?
MD = 0.100M
VD = 250. mL
MCVC= MD VD
0.500 M x VC = 0.100 M x 250. mL
VC = 0.100 M x 250. mL
(0.500 M)
VC = 50.0 mL of 0.500 M CuCl2 stock solution
Dilute 50.0 mL of 0.500 M CuCl2 stock solution with enough water to
prepare 250. mL of solution.
Diluting a Solution
(a)
Measure volume of concentrated stock
(b)
Transfer into volumetric Flask
(c)
Fill to line with water.