Calculating molar solutions a document prepared by Bright Kumwenda

1. Mass
Volume
Concentration

2.  Describe how to calculate mass using mole
concept
 Describe how to dilute a molar solution from
a stock solution
 Describe the calculation of molar volume of a
gas at Standard Temperature and
Pressure(STP) and at Room Temperature and
Pressure(RTP)

3. The mole (or mol) represents a certain number
of objects.
The amount of a substance that contains the
same number of entities as there are atoms in
12 g of carbon-12.
Exactly 12 g of carbon-12 contains 6.02x 10 23
atoms.

4.  One mole of H2O molecules contains 6.022 x
10 23 molecules.
 1 mole contains 6.02 x 10 23 entities
(Avogadro’s number)
 Mass of a mole of particles = mass of 1
particle x 6.022 x 1023

5. The mass of an atom in amu is numerically the
same as the mass of one mole of atoms of
the element in grams.
One atom of sulfur has a mass of 32.07 amu.
One mole of S atoms has a mass of 32.07 g.

6.  • For compounds, the molecular mass (in
amu) is numerically
the same as the mass of one mole of the
compound in grams. Calculate the molecular
mass of a compound as the
sum of the atomic masses of its elements.
• Molecular mass H2O = (2 x atomic mass of
H) + atomic mass of O
= 2(1.008 amu) + 16.00 amu = 18.02 amu
• So, one mole of water (6.022 x 10 23
molecules) has a mass of 18.02 g.

7.  •The molar mass (M) of a substance is the
mass of one mole of entities (atoms,
molecules, or formula units) of the substance.
• Molar mass has units of grams per mole
(g/mol)

8. •Use the molar mass of an element or
compound to convert a given number of
moles to mass:
•We can do the reverse and convert any mass
in grams to the number of moles:
•Use Avogadro’s number to convert moles of
substance to the number of entities

9.  How many grams of silver, Ag, are in 0.0342
mol Ag?
 To convert moles Ag to grams Ag, use the
molar mass of Ag from the periodic table.

10.  How many atoms of Ag are in 0.0342 mol
Ag? To convert moles to number of atoms,
use Avogadro’s number.

11.  Molarity” is a concentration term. It refers to
the number of moles of a substance per liter
of solution.
 If you recall, a “mole” is the quantity of
substance that contains 6.02 x 1023
(Avogadro’s number) items.
 You may also recall that the atomic or
formula weight of a substance, in grams,
contains 1 mole (6.02 x 1023 units) of that
substance.

12.  It is relatively easy to work out calculations
for the number of moles of substance into
the concentration term “molarity”.
 To calculate the molarity of a solution, simply
divide the number of moles of substance it
contains (solute) by the volume of the
solution, in liters.
 One mole contains Avogadro’s Number
(6.022 x 1023)

13.  A mole is the amount of a substance of a system
which contains as many elementary entities as
there are atoms in 0.012kg (or12g) of Carbon-12
 A mole is that quantity of a substance whose mass
in grams is the same as its formula weight
 E.g. Fe55.85
 Iron has an atomic mass or 55.85g mol-1, so one
mole of iron has a mass of 55.85g

14.  To dilute a solution scientists add more
solvent (usually water) without the addition of
more solute.
 This lowers the overall concentration of the
solution.
 Review the formula M1V1 = M2V2
or more generally
C1V1 = C2V2

15. You have made 53.3 mL a stock solution of 1.5
molar solution of potassium chloride. Your
laboratory protocol requires a 0.8 molar
potassium chloride solution for the procedure.
Prepare the 0.8 molar solution needed by
dilution.
 M1V1 = M2V2
 1.5 M x 53.3 mL =0 .8 x V2
 1.5 M x 53.3 mL =
0.8 M
99.938 ml = V2

16.  You have a stock solution of 95% ethanol. Your
laboratory protocol requires 25 mL a solution of
20% ethanol for the procedure. How would you
prepare the 25ml 20% ethanol solution needed by
dilution?
 Calculate the volume of stock solution needed:
 M1V1 = M2V2
 95% x V1 = 20% x 25 mL
 V1 = 20% x 25 mL
95%
 V1 = 5.26mL

17.  Determine the amount of water to add to the
solution to make the 25 mL needed.
 25ml - 5.26ml = 19.74ml
 3. Make the solution.
 X = 5.26 ml of 95% ETOH + 19.74 ml H2O

18.  What is Molar Volume?
 The Molar Volume, represented by Vm, is the
volume occupied by one mole of a substance
which can be a chemical element or a
chemical compound at Standard Temperature
and Pressure (STP).
 One mole of any gas at a particular
temperature and pressure has fixed volume
and known as its Molar gas volume.

19.  Where Vm is the volume of the substance.
 The standard temperature used is 273 Kelvin
or 0oC, and standard pressure is 1
atmospheres, i.e., 760 mm Hg.
Experimentally, one mole of any gas occupies
a volume of 22.4 liters at STP. The equation
can be expressed as
 1 mole of gas at STP = 22.4 liters of gas.

20.  Calculate the volume of carbon dioxide gas, CO2,
occupied by (a) 5 moles and (b) 0.5 moles of the
gas occupied at STP.
 Solution:
 a) Volume of CO2
= number of moles of CO2 × 22.4 L
= 5 × 22.4
= 112 L
b) Volume of CO2
= number of moles of CO2 × 22.4 L
= 0.5 × 22.4
= 11.2L

21.  Calculate the number of moles of ammonia
gas, NH3, in a volume of 80 L of the gas
measured at STP.
 Solution:
 Volume of gas = number of moles × 22.414
L/mol
 Number of moles=Volume of gas
22.414

22. 
 No of moles= 80L
22.414
=3.6