<< Chapter < Page
  Stoichiometry   Page 1 / 1
Chapter >> Page >

Molality (m) is also a measurement of molar concentration like molarity (M). We have seen that molarity(M) is a convenient measurement of the concentration of solution as it allows us to directly compute moles of solute present in the solution. There is, however, a problem in reporting concentration of a solution in terms of molarity. Recall that it is equal to numbers of moles of solute divided by volume of solution in litres. The molar ratio has volume of the solution in the denominator. This means that molarity(M) of a given solution will change with temperature as volume of solution will change with temperature. This is a major handicap as reported concentration needs to be reliable and constant anywhere irrespective of temperature. In this backdrop, molality(m) is measurement of concentration aiming to remove this shortcoming associated with measurement in molarity (M).

Molality(m) also differs to other measurements in yet another important aspect. It involves the ratio of measurement of solute and solvent – not that of solute and solution. This difference is important to be kept in mind while computing quantities and converting measurement units from one to another.

The major objective of this module is to develop skills to convert measurement of concentration from one measurement type to another.

Molality of a solution with respect to solute is defined as :

Molality(m) = Moles of solute (B) Mass of solvent (A) in Kg

m = n B w Akg

Its unit is moles/ kg. In case we consider mass of solvent in gm, then the expression of molality is given as :

m = n B g A X 1000 = Milli-moles of B g A

If mass of solvent of a solution of known molality is known, then number of moles of solute is obtained as :

Moles of solute, B = Molality X Mass of solvent in kg

Similarly,

Milli-moles of B = Molality X Mass of solvent (A) in gm

Problem : 11.7 gm of sodium chloride is dissolved in 400 ml of water. Find molality of the solution.

Solution : Here,

Moles of sodium chloride = 11.7 23 + 35.5 = 11.7 58.5 = 0.2

Mass of the solvent = 400 X 1 = 400 g m = 0.4 k g

Molality, m = 0.2 0.4 = 0.5 m

Molality in terms of molarity and density of solution

Molality and Molarity are linked to each other through density of solution. Beginning with the definition of molarity, a solution of molarity “M” means that 1 litre of solution contains “M” moles of solute. If the density of the solution is “d” in gm/cc, then

Mass of 1 litre solution in gm = 1000 d

Mass of the solute in gm in 1 litre solution = nos of moles X molecular weight = M M O

Mass of the solvent in gm in 1 litre solution = 1000 d M M O

We need to calculate mass of solvent in kg to calculate molality(m) :

Mass of the solvent in kg in 1 litre solution = 1000 d M M O / 1000

Hence, molality,

m = n B W Akg = M 1000 d M M O 1000 = 1000 M 1000 d M M O

We should note that "density of solution (d)" and "strength of solution (S)" differ. Density of solution (d) is ratio of mass of solution (solute + solvent) in gm and volume of solution in cc. It has the unit of gm/cc. On the other hand, strength of solution (S) is ratio of mass of solute in gm and volume of solution in litre. It has the unit of gm/litre.

Problem : The density of 3M sodium thiosulphate (Na2S2O3) solution is 1.25 gm/cc. Calculate molality of Na + and S 2 O 3 ions.

Solution : We can use the formula to calculate molality of the sodium thiosulphate :

m = 1000 M 1000 d M M O = 1000 X 3 1000 X 1.25 3 X 2 X 23 + 2 X 32 + 3 X 16

m = 3000 1250 474 = 3000 776 = 3.866

Alternatively , we can proceed with the basic consideration in place of using formula Since molarity of solution is 3M, it means that 1 litre of solution contains 3 moles of sodium thiosulphate. We can use density to find the mass of the 1 litre solvent.

Mass of solution = 1000 X 1.25 = 1250 g m

Mass of 3 moles of sodium thiosulphate = 3 M N a 2 S 2 O 3 = 3 X 2 X 23 + 2 X 32 + 3 X 16

Mass of 3 moles of sodium thiosulphate = 3 X 46 + 64 + 48 = 3 X 158 = 474 g m

Mass of solvent = 1250 474 = 776 g m = 0.776 k g

Molality of sodium thiosulphate = 3 0.776 = 3.866 m

We are, however, required to calculate molality of ions. We see that one mole is equivalent to 2 moles of sodium ion and 1 mole of thiosulphate ion :

Na 2 S 2 O 3 = 2 N a + + S 2 O 3

Hence,

Molality of Na + = 2 X 3.866 = 7.732 m

Molality of S 2 O 3 = 3.866 m

Density of solution in terms of molarity and molality

Working on the relation of molality developed in previous section :

m = 1000 M 1000 d M M O

1 m = 1000 d M M O 1000 M = d M M O 1000

d = M 1 m + M O 1000

Problem : The molality and molarity of a solution of sulphuric acid are 90 and 10 respectively Determine density of the solution.

Solution : Using relation :

d = M 1 m + M O 1000

d = 10 1 90 + 98 1000 = 10 X 0.011 + 0.98 = 10 X 0.991 = 9.91 g m / c c

Molality in terms of density of solution and strength of solution

We need to know the moles of solute and mass of solvent in kg to determine molality. Now, strength of solution (S) is equal to mass of the solute in gm in 1 litre of solution. Hence,

Mass of the solute in gm in 1 litre of solution = S

Moles of the solute = S M O

Mass of 1 litre solution in gm = 1000 d

Mass of the solvent in gm in 1 litre of solution = 1000 d S

Mass of the solvent in kg in 1 litre of solution = 1000 d S 1000

The molality is :

m = n B W Akg = S M O 1000 d S 1000 = 1000 S M O 1000 d S

Problem : A solution has 392 gm of sulphuric acid per litre of solution. If the density of the solution is 1.25 gm/cc, find molality of the solution.

Solution : Using relation :

m = 1000 S M O 1000 d S = 1000 X 392 98 X 1000 X 1.25 392 = 4.73 m

Molality in terms of mole fraction and molecular weight

Molality is defined as :

m = n B W A k g = n B g A X 1000 = n B n A M A X 1000

n B = m n A M A 1000

On the other hand, mole fraction with respect to solute B is given by :

χ B = n B n A + n B

Substituting for nA, we have :

χ B = m n A M A 1000 n A + m n A M A 1000 = m M A 1000 + m M A

1000 χ B + m M A χ B = m M A

m M A 1 χ B = 1000 χ B

m = 1000 χ B 1 χ B M A

Similarly, we can express molality in terms of mole fraction with respect to solvent (A) as :

m = 1000 1 χ A χ A M A

Questions & Answers

where we get a research paper on Nano chemistry....?
Maira Reply
nanopartical of organic/inorganic / physical chemistry , pdf / thesis / review
Ali
what are the products of Nano chemistry?
Maira Reply
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
learn
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
learn
Google
da
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Bhagvanji
hey
Giriraj
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
revolt
da
Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
ya I also want to know the raman spectra
Bhagvanji
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
Damian
yes that's correct
Professor
I think
Professor
Nasa has use it in the 60's, copper as water purification in the moon travel.
Alexandre
nanocopper obvius
Alexandre
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
LITNING Reply
what is a peer
LITNING Reply
What is meant by 'nano scale'?
LITNING Reply
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
what is differents between GO and RGO?
Mahi
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
Rafiq
if virus is killing to make ARTIFICIAL DNA OF GRAPHENE FOR KILLED THE VIRUS .THIS IS OUR ASSUMPTION
Anam
analytical skills graphene is prepared to kill any type viruses .
Anam
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
Hafiz
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
why?
Adin
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
Got questions? Join the online conversation and get instant answers!
Jobilize.com Reply

Get the best Algebra and trigonometry course in your pocket!





Source:  OpenStax, Stoichiometry. OpenStax CNX. Jul 05, 2008 Download for free at http://cnx.org/content/col10540/1.7
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Stoichiometry' conversation and receive update notifications?

Ask