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## Mole Concept-II, Chemistry tutorial

:IntroductionWe are familiar that, the mole was stated and the mole concept employed in calculations. The mole concept is specifically helpful in predicting yield and computing yield from experiments. This is very significant for industrial or laboratory procedures that comprise reversible reactions. Yield and yield percent computations let chemists to evaluate the efficiency of chemical processes and look for ways of enhancement where possible and applicable.

:Mole Concept in SolutionsThe molar and molal solutions:

A solution which includes one mole of solute in 1.0 dm

^{3}of the solution is termed as a molar solution. A molar solution is made by dissolving one mole of the solute in small amount of the solvent and then makes the solution up to 1.0 dm^{3}in a standard volumetric flask. A molar solution can as well be made from amount less than or equivalent to one mole of the solute.Illustration:

It is needed to prepare 250.00 cm

^{3}of 1.0 mol dm^{-3}solution of glucose (C_{6}H_{12}O_{6}).Calculate:

a) Mole of glucose needed.

b) Mass of glucose needed.

Solution:

Molar mass of glucose = 6(12) + 12(1) + 6(16) g

= 180 g

For a molar solution of glucose 1dm

^{3}includes 1.0 mol250 cm

^{3}will have = (1.0 x 250)/(1000) = 0.25 mola) 0.25 mol glucose is needed

b) Mass of glucose = mol x molar mass

= 0.25 x 180

= 45 g

A molal solution includes 1 mole in 1 kg of the solvent

A molal solution is made by dissolving 1 mole of the solute in 1000g of solvent. The solute and solvent are weighed and mixed in the affirmed proportions. The final volume of solution is immaterial.

Note:

From the statements and processes of preparation, both solutions are not similar. The preparation of solutions in volumetric (that is, titrimetric) analysis is done in the standard volumetric flasks and solution concentrations are represented in mol dm

^{-3}Not all the solutions are molar solutions. The concentration of a solution is computed from the amount of solute and the volume of solution.

Computation of mole and mass in solution:

If the concentration of a solution is represented in mol dm

^{-3}, it is possible to compute the amount in moles and grams of the solute in a volume of solution. You should keep in mind that however a solution consists of one concentration, the amount of solute will be dissimilar for different volumes of the solution. A volume of sea water will taste similar whether you test a cup of it or a bucket or a drop. The amount of salt you recover from sea water though based on the volume of sea water evaporated.Example:

Compute the amount of sodium chloride recoverable from (a) 1.0 dm

^{3}(b) 10 dm^{3}(c) 3 drums of sea water. (1 drum - 50 dm^{3}). Concentration of sodium chloride in the sea water is 0.43 mol dm^{-3}. Na = 23, Cl = 35.5 Compute the amount in mole and g dm^{-3}.Solution:

i)

a) 1.0 dm

^{3 }consists of 0.43 mol/NaCl.b) 10.0 dm

^{3}consists of 0.43 x 10 = 4.3 mol NaClc) 3 drums = 3 x 50 dm

^{3}= 150 dm^{3}150.0 dm

^{3}consists of 0.43 x 150 = 64.5 mol NaClii)

a) Amount in g = mole x molar mass

= 0.43 x 58.5

= 25.16g

b) Amount in g = 4.3 x 58.5

= 251.6g

c) Amount = 64.5 x 58.5 g

= 3773.3g

= 3.77 kg

:The Mole Concept in Volumetric AnalysisDetermination of concentration of a solution:

The solution of unknown concentration is titrated by a standard solution (a solution of acknowledged concentration).

Example:

25.0 cm

^{3}of 0.10 mol dm^{-3}sodium hydroxide solution needs 21.5 cm^{3 }ethanoic acid solutions for complete neutralization by employing phenolphthalein indicator. Compute the concentration of the ethanoic acid solution in (a) mol dm^{-3}and (b) g/dm^{3}. Equation of the reaction is NaOH^{+}CH_{3}COOH → CH_{3}COONa + H_{2}OSolution:

Mole ratio NaOH: CH

_{3}COOH = 1:1Mole NaOH = [Conc (mol dm

^{-3})/1000] x V (cm^{3})= [(0.10 x 25)/1000] = 2.5 x 10

^{-3}As mole ratio is 1:1

Mole CH

_{3 }COOH = 2.5 x 10^{-3}2.5 x 10

^{-3}mole CH_{3}COOH is in 21.5 cm^{3}of solutionThus, [(2.5 x 10

^{-3})/21.5] x 1000 mol CH_{3}COOH is in 1.0 dm^{3}of solution.Concentration (mol dm

^{-3}) = 0.116Concentration (g/dm

^{3}) = mol dm^{-3}x molar mass= 0.116 x 60

= 6.96 g dm

^{-3}Determination of percentage purity:

The mole concept can be applied in the volumetric analysis for the determination of percentage purity of an acid or an alkali sample.

Illustration:

5.0 g of sodium hydroxide dissolved in the 1.0 dm

^{3 }of solution. 25.0cm^{3}of the solution needs 24.9 cm^{3 }of a 0.10 mol dm^{-3}trioxonitrate (v) acid for complete neutralization by employing methyl orange indicator. Compute the percentage purity of the sodium hydroxide sample. The equation of reaction is:NaOH (aq) + HNO

_{3}(aq) → NaNO_{3}(aq) + H_{2}O (l)Mole ratio NaOH: HNO

_{3}= 1:1Mole HNO

_{3}/24.9 cm^{3}= (24.9 x 0.10)/1000= 2.49 x 10

^{-3}Thus, mole NaOH/25.0 cm

^{3 }= 2.49 x 10^{-3}Mole NaOH/1000 cm

^{3}= (2.49 x 10^{-3}x 1000)/25= 0.0996 mol dm

^{-3}g/dm

^{3}(NaOH) = .0996 x molar mass (NaOH)= .0996 x 40

= 3.984 g

Percentage purity = (Amount computed/Amount dissolved) x 100

= (3.984/50) x 100

= 79.68

Mole concept in solution dilution:

Numerous times it is needed to make a dilute solution from a relatively concentrated solution of the similar substance in the similar solvent. Mole concept allows for computation to know how much solvent should be added to get the required concentration. Dilution becomes the mere way of making dilute solution of common acids that are available commercially as concentrated acids example: H

_{2}SO_{4}, HCl and HNO_{3}.Illustration:

A bottle of commercial tetraoxosulphate (vi) acids reads 98 percent w/w, specific gravity 1.84, molar mass 98 g.

Compute the volume of this commercial acid sample needed to prepare 2.0 dm

^{3}of a 0.05 mol dm^{-3}solution.Solution:

98% w/w means 98g H

_{2}SO_{4}in 100g of solution.98g H

_{2}SO_{4}are in 100 g of commercial acid98g H

_{2}SO_{4 }are in (100/1.84) cm^{3 }of concentrated acidIt is noted that specific gravity = (mass of substance/mass of an equivalent volume of water)

= (mass of substance/volume of substance)

This signifies that specific gravity is numerically similar as density. Remember,

Density = mass/volume

Thus, Volume = mass/density

98g H

_{2}SO_{4}are in (100/1.84) cm^{3}Thus, [(98 x 1000 x 1.84)/100] H

_{2}SO_{4}are in 1000 cm^{3}1803.2 g H

_{2}SO_{4}are in 1000cm^{3}Concentration (mol dm

^{-3}) = [(g/dm^{3})/molar mass]= 1803.2/98

= 18.4 mol dm

^{-3}Mole of acid needed for the preparation of 2.0 dm

^{3}of 0.05 mol dm^{-3}solution= mol dm

^{-3}x volume (dm^{3})= 0.05 x 2 = 0.10 mol dm

^{-3}For concentrated acid:

18.4 mol are present in 1000 cm

^{3}0.10 mol will be present = (0.10/18.4) x 1000

= 5.435 cm

^{3}This latter portion of computation can be done by using the formula:

M

_{c}V_{c}= M_{d}V_{d}M

_{c}: concentration of commercial acidV

_{c}: volume of commercial acidM

_{d}: concentration of dilute acidV

_{d}: volume of dilute acid.On substituting,

18.4 x V

_{c}= 0.05 x 2000 cm^{3}V

_{c}= (0.05 x 2000)/18.4 cm^{3}= 5.435 cm

^{3}It will be noted that M

_{c}V_{c =}millimoles of concentration acidM

_{d}V_{d}= millimoles of dilute acidBy equating the millimoles, the volume of concentrated acid needed is computed.

:The Mole Concept in ElectrolysisThe mole concept and the Faraday:

Throughout electrolysis chemical reactions take place and substances are liberated or discharged at the electrodes. The Faraday is quantity of charge carried via 1 mole (6.02 x 10

^{23}) of electrons. A Faraday discharges or liberates 1 mole of a univalent ion like H^{+}H

^{+}+ e → HA mole H

^{+}will need a mole of electrons to form a mole of H atoms.Illustration: Compute the charge on an electron given that Faraday's constant is 96500 coulombs.

By definition:

96500 = 6.02 x 10

^{23}x e, here e is the electronic chargee = 96500/(6.02 x 10

^{23}) = 1.603 x 10^{-19}coulombsThe mole concept and the efficiency of an electrolytic method:

This can be computed by comparing amount of reaction which occurred at the electrode by the quantity of electricity passed. In such computations the Faraday is employed as a base unit of electricity.

Illustration:

4,825 coulombs of electricity passed in the electrolysis procedure. The volume of hydrogen gas discharged was 0.50 dm

^{3}at S.T.P. Compute the efficiency of the electrolysis method. (Take Faraday constant = 96500 coulombs).(Hint: 1 Faraday of electricity must liberate 1.0g H

_{2}gas). Molar volume of gas = 22.4 dm^{3}at S.T.P.)Solution: 1 Faraday = 6.02 x 10

^{23}electrons = 96500 coulombsFor this procedure:

1 Faraday discharges = 1.0g H

_{2}gas1 Faraday discharges = (1.0/2.0) x 22.4 dm

^{3}H_{2}at STP= 11.2 dm

^{3}4825 coulombs = 4825/96500 Faraday = 0.05 Faraday

1 Faraday discharges 11.2 dm

^{3}at STP.05 Faraday must discharge.05 x 11.2 = 0.56 dm

^{3 }at STPThe efficiency of the procedure = Volume H

_{2}liberated/Theoretical volume= 0.50/0.56

= 0.89

Percentage efficiency = 89 percent

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