<< Chapter < Page Chapter >> Page >

Back titration and dilution

While dealing dilution in titration calculation, we need only to remember that dilution does not change geq or meq of a solution.

Problem : 2.75 gram of a sample of dolomite containing C a C O 3 and M g C O 3 is dissolved in 80 ml 1N HCl solution. The solution is then diluted to 250 ml. 25 ml of this solution requires 20 ml of 0.1 N NaOH solution for complete neutralization. Calculate percentage composition of the sample.

Solution : We shall first determine the amount of HCl required to react with the sample by using back titration data. Here, we make use the fact that dilution does not change meq of a solution.

meq of 25 ml of diluted excess HCl solution = meq of NaOH solution = 0.1 X 20 = 2

meq of 250 ml of diluted excess HCl solution = 20

total meq of HCl = N V = 80 X 1 = 80

meq of HCl used to react with dolomite = 80 20 = 60

Mass of HCl used in reaction with dolomite is calculated using expression of meq :

m e q = g E X 1000 = x g M O X 1000

g = m e q X M O 1000 x = 60 X 36.5 1000 X 1 = 2.19 g m

In order to find the composition, we apply mole concept. The reactions involved are :

C a C O 3 + 2 H C l C a C l 2 + C O 2 + H 2 O

M g C O 3 + 2 H C l M g C l 2 + C O 2 + H 2 O

Let mass of C a C O 3 in the sample is x gm. Then mass of M g C O 3 is 2.75-x gm.

x M C a C O 3 moles of C a C O 3 = x 100 moles of C a C O 3 = 2 x 100 moles of HCl

mass of HCl required for C a C O 3 = 2 x X M H C l 100 = 2 x X 36.5 100 = 73 x 100 = 0.73 x g m

Similarly for M g C O 3 ,

mass of HCl required for M g C O 3 = 2 2.75 x M H C l 84 = 2 X 2.75 x 36.5 84 = 73 2.75 x 84 = 2.39 0.87 x g m

total HCl required = 0.73 x + 2.39 0.87 x = 2.19

0.14 x = 0.2

x = 1.43 g m

% mass of C a C O 3 = 1.43 2.75 X 100 = 52

% mass of M g C O 3 = 100 52 = 48

Back titration and decomposition

Back titration is analyzed using geq or meq concept. On the other hand, decomposition is analyzed using mole concept. We combine two concepts by converting final geq or meq data of back titration into either mass or moles.

Problem : 4.08 gm of a mixture of BaO and an unknown carbonate M C O 3 is heated strongly. The residue weighs 3.64 gram. The residue is then dissolved in 100 ml of 1 N HCl. The excess acid requires 16 ml of 2.5 NaOH for complete neutralization. Identify the metal M, if atomic weight of Ba is 138.

Solution : We need to determine atomic weight of M to identify it. From the question, we see that first part involves decomposition, which can be analyzed using mole concept. Note that difference of mass of mixture and residue is mass of C O 2 . On the other hand, analysis of back titration gives us meq of HCl required for neutralization of residue. To combine meq data with decomposition data, we shall convert required meq in milli-moles.

total meq of HCl = N V = 1 X 100 = 100

meq of excess HCl = N V = 2.5 X 16 = 40

meq of HCl used for neutralization of residue = 100 40 = 60

milli-moles of HCl for neutralization of residue = x X m e q = 1 X 60 = 60

Now, we analyze the data which is given for decomposition of mixture. On heating, BaO is not decoposed. Only M C O 3 is decomposed as :

M C O 3 = M O + C O 2

Carbon dioxide is released and is not part of residue. The amount of C O 2 is equal to difference of mass of mixture and residue. Hence, moles of C O 2 released are :

n C O 2 = 4.08 3.64 12 + 2 X 16 = 0.44 44 = 0.01 m o l e

Applying mole concept, moles of MO in the residue is 0.01 mole i.e. 10 milli-moles. Now, MO reacts with HCl as :

M O + 2 H C l M C l 2 + H 2 O

Moles of HCl required for MO is 2X0.01 moles = 20 milli-moles of HCl. But total milli-moles that reacted with residue is 60 milli-moles. Hence, milli-moles of HCl that reacted with BaO is 60-20 = 40 milli-moles of HCl. Applying mole concept,

B a O + 2 H C l B a C l 2 + H 2 O

The milli-moles of BaO is half of that of HCl. Hence, millimoles of BaO in the mixture is 20 milli-moles.

amount of BaO in the mixture = milli-moles X M B a O 1000 = 20 X 138 + 16 1000 = 3.08 g m

amount of MO in the residue = 3.64 3.08 = 0.56 g m

Using formula :

g = milli-moles X M M O 1000 = 10 X M M O 1000

M M O = 100 g = 100 X 0.56 = 56

Clearly, atomic weight of M is 56-16 = 40. The element, therefore, is calcium.

Back titration and multiple neutralizations

The concentration of titrant used to determine the excess reactant is not directly given. We make use of subsequent neutralization data to ultimately determine the concentration of titrant.

Problem : A sample of M n S O 4, 4 H 2 O (molecular wt : 223) is heated in air. The residue is Mn 3 O 4 , whose valency and molecular weight are 2 and 229 respectively. The residue is dissolved in 100 ml of 0.1 N F e S O 4 solution containing dilute sulphuric acid. The resulting solution reacts completely with 50 ml of K M n O 4 solution. 25 ml of the K M n O 4 solution requires 30 ml of 0.1N F e S O 4 solution for complete oxidation. Calculate the weight of M n S O 4, 4 H 2 O in the given sample.

Solution : The excess of F e S O 4 completely reacts with 50 ml of K M n O 4 solution of unknown concentration. We need to know this concentration. From the data of complete oxidation, however, we can know concentration. Let 1 and 2 subscripts denote K M n O 4 and F e S O 4 , then

N 1 V 1 = N 2 V 2

N 1 X 25 = 0.1 X 30

N 1 = 3 25 = 0.12 N

Now, considering back titration,

total meq of F e S O 4 solution = 0.1 X 100 = 10

meq of excess F e S O 4 solution = 0.12 X 50 = 6

meq of F e S O 4 solution used for residue = 10 6 = 4

meq of M n 2 O 3 in the residue = 4

Mass of Mn 3 O 4 is determined using meq expression :

m e q = g E X 1000 = x g M O X 1000

g = m e q X M O 1000 x = 4 X 229 1000 X 2 = 0.458 g m

The decomposition of M n S O 4, 4 H 2 O takes place as :

3 M n S O 4, 10 H 2 O M n 3 O 4 + 3 S O 2 + 10 H 2 O

Applying mole concept, the amount of M n S O 4, 4 H 2 O is :

Mass of M n S O 4, 4 H 2 O = 3 X 223 229 X 0.458 = 1.338 g m

Questions & Answers

the art of managing the production, distribution and consumption.
Satangthem Reply
what is economics
Khawar Reply
okk
damfash
marginal utility is the additional satisfaction one derives from consuming additional unit of a good or service.
Fred
It's the allocation of scarce resources.
Fred
marginal utility please?
Dishan
marginal utility is the additional satisfaction one derives from consuming additional unit of a good or service.
Fred
I know the definition, but I don't understand its meaning.
Dishan
what is the must definition of economic please?
Nurudeen
demand lfs
Alpha
Economics is derived from the word Oikonomia which means management of household things. Thus, Economics is a study of household things with the constrains of allocating scare resources.
Dishan
what is Open Market Operation
Adu Reply
dominating middlemen men activities circumstances
Christy Reply
what Equilibrium price
Adji Reply
what is gap
mirwais
who is good with the indifference curve
Dexter
What is diseconomic
Alixe Reply
what are the types of goods
WARIDI
how can price determination be the central problem of micro economics
simon Reply
marginal cost formula
Nandu Reply
you should differentiate the total cost function in order to get marginal cost function then you can get marginal cost from it
boniphace
What about total cost
Foday
ok
Foday
how can price determination be the central problem if micro economics
simon
formula of cross elasticity of demand
Theresia Reply
what is ceteris paribus
Priyanka Reply
what is ceteris parabus
Priyanka
Ceteris paribus - Literally, "other things being equal"; usually used in economics to indicate that all variables except the ones specified are assumed not to change.
Abdullah
What is broker
scor
land is natural resources that is made by nature
scor
What is broker
scor
what is land
kafui
What is broker
scor
land is natural resources that is made by nature
scor
whats poppina nigga turn it up for a minute get it
amarsyaheed Reply
what is this?
Philo
am from nigeria@ pilo
Frank
am from nigeria@ pilo
Frank
so
owusu
what is production possibility frontier
owusu
it's a summary of opportunity cost depicted on a curve.
okhiria
please help me solve this question with the aid of appropriate diagrams explain how each of the following changes will affect the market price and quantity of bread 1. A
Manuela Reply
please l need past question about economics
Prosper Reply
ok let me know some of the questions please.
Effah
ok am not wit some if den nw buh by tommorow I shall get Dem
adepojurafiu
Hi guys can I get Adam Smith's WEALTH OF NATIONS fo sale?
Ukpen
hello I'm Babaisa alhaji Mustapha. I'm studying Economics in the university of Maiduguri
Babaisa
okay
Humaira
my name is faisal Yahaya. i studied economics at Kaduna state university before proceeding to West African union university benin republic for masters
Faisal
Hi guys..I am from Bangladesh..
Mannan
Wat d meaning of management
igwe Reply
disaster management cycle
Gogul Reply
cooperate social responsibility
igwe
Fedric Wilson Taylor also define management as the act of knowing what to do and seeing that it is done in the best and cheapest way
OLANIYI
Difference between extinct and extici spicies
Amanpreet Reply
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