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CONCENTRATIONS (MOLARITY), Lecture notes of Chemistry

Key CollegeChemistry

this document will help to understand and solve exercise regarding molarity

Typology: Lecture notes

2019/2020

Uploaded on 08/26/2021

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WORKSHEET # 2
CLASS IX
CHEMISTRY
CHAPTER-7 SOLUTION AND SUSPENSION
2. Molality (m): It is defined as number of moles of solute per kilogram of
solvent.
Supporting formulae:
1. Number of moles =
Given mass of solute
Given formula mass of solute
2. To convert (x)g to kg:
No. of kg = (x)g ×
1kg
1000 g
1kg = 1000g⸫
3. Mass of solvent(kg) = density(g/ml) × volume(ml)
which can be simplified as: Mass = d × v
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WORKSHEET # 2

CLASS IX

CHEMISTRY

CHAPTER-7 SOLUTION AND SUSPENSION

2. Molality (m): It is defined as number of moles of solute per kilogram of

solvent. Supporting formulae:

1. Number of moles =

Given mass of solute Given formula mass of solute

  1. To convert ( x )g to kg: No. of kg = ( x )g × 1 kg 1000 g ⸫1kg = 1000g
  2. Mass of solvent(kg) = density(g/ml) × volume(ml) which can be simplified as: Mass = d × v

Example 1: Calculate the molality of a solution prepared from 29.22 grams of

NaCl in 2.00 kg of water.

Data:

Mass of NaCl (solute) = 29.22 gm Mass of water (solvent) = 2.00 kg Molality =?

Solution:

Note: Convert grams of NaCl to moles of NaCl Formula:

Number of moles =

Given mass of solute Given formula mass of solute

29.22 g

58.44 g / mol =^ 0.50 moles

(Gram formula mass of NaCl = 22.99 + 35.45 = 58.44 g/mol)

Formula: m = 0.50 moles 2.00 kg =^ 0.25 moles/kg or molal or m

m =

0.75 moles

0.5 kg =^ 1.5 mole/kg or molal or m

Result: The molality of the NaOH solution is found to be 1.5 m

  1. 49.8 grams of KI is dissolved in 1.00 kg of solvent. What is the molality? [AT.Mass K = 39, I = 127]
  2. Calculate the molality when 75.0 grams of MgCl2 is dissolved in 500.0 g of solvent. [AT.Mass Mg = 24.3, Cl = 35.5]

Question No. 1 2 3 4 Answers 0.300 m 1.57 m 0.280 m 0.444 m

4. Mole Fraction (X): It is the number of moles of a component divided by

the total number of moles of a solute and solvent. Because it is a ratio, mole fraction is a unitless expression. The mole fraction of all components of a solution, when added together, will equal 1. X = Moles of component Total moles of solution Xsolute = moles of solute total moles of solution

Xsolvent =

moles of solvent total moles of solution Where Xsolute + Xsolvent = 1

Example 2: 0.100 mole of NaCl is dissolved into 100.0 grams of pure H 2 O. What is the mole fraction of NaCl?

Data:

Mole of NaCl (solute) = 0.100 mole Mass of H 2 O (solvent) = 100.0 g Mole fraction (X) =?

Solution:

Note: (a) Calculate moles of solvent. (b) Calculate mole fraction of NaCl. (a) Moles of solvent = mass of solvent Gram formula mass of solvent

100 g 18 g / mol

= 5.56mole

(Gram formula mass = H 2 O = 2 + 16 = 18 g/mol)

(b)

XNaCl =

Moles of NaCl Total moles of solution

0.100 mole 5.56+0.100 mole

Result: Mole fraction of NaCl is found to be 0.

  1. A solution has been prepared by dissolving 2 moles of ethyl alcohol in 8 moles of water. Calculate the mole fractions of [a]ethyl alcohol and [b]water in this solution.
  2. Find the mole fraction of [a]Methanol (CH 3 OH) and [b]water in a solution prepared by dissolving 4.5 g of alcohol in 40 g of H 2 O.
  • Question No.
  • Answers (a)0.
    • (b)0.
      • (a)0.
      • (b) 0.
        • A= 0.
        • B= 0.
        • C= 0.