Molarity of Solutions

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Molarity of Solutions Dr Graeme Jones
LECTURE 5
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Molarity of Solutions

• Definition A 1 molar (1M) solution is obtained
  when 1 mole of a compound is dissolved in 1 dm3
  of a solvent (1 dm3 1000 cm3 1 litre). The
  concentration of such a solution is 1 mol dm-3.
• The relationship between the number of moles (n),
  the molarity (M) and the volume V is
• n M x V

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Molarity of Solutions

• n M x V can be used to answer most questions
  regarding molarity.
• 3 types of problem are usually encountered
• (i) calculating the molarity of a solution
• (ii) calculating what is needed to produce a
  solution of specified molarity
• (iii) dealing with dilution of solutions

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Calculating the Molarity of Solutions

• From n M x V molarity can be calculated by
  dividing the number of moles by the volume.
  Remember we need to make sure the volume is
  converted into dm3, so if working in cm3 then we
  need to factor the volume by 1000 cm3 dm-3.
• What is the molarity of the following solutions?
• (a) 1 mole of HCl in 100 cm3 water,
• (b) 0.5 moles of H2SO4 in 1000 cm3 water,
• (c) 10 g KCl in 25 cm3 water

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Mass required to produce a specific molarity

• In these calculations it is necessary to
  determine the number of moles of the substance
  required and then convert that use the molecular
  mass to a mass in g.
• (a) What mass of sodium chloride (NaCl) must be
  dissolved in 1 dm3 of water to produce (i) a 1M
  solution, (ii) a 0.5M solution?
• (b) What mass of potassium permanganate (KMnO4)
  is needed to prepare 250 cm3 of (i) a 1M
  solution, (ii) a 0.5M solution, (iii) a 0.0385M
  solution?

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Dilution of Solutions

• When a solution is diluted, the molarity changes
  (decreases) but the number of moles of solute
  remains the same. If we dilute a solution of
  molarity M1, volume V1, the resulting solution
  will have a molarity and volume M2, V2 given by
• M1 x V1 M2 x V2
• In most situations three of the quantities will
  be known, and you will be asked to calculate the
  fourth.
• Example calculate the volume of a 0.0155M HCl
  solution that should be used to prepare 100 cm3
  of 5.23 x 10-4M HCl solution.

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