Concentration of Acids and Alkalis

Sodium hydroxide

Concentration of Acids and Alkalis

• Quantity of solute can be measured in grams or moles: g dm^-3 or mol dm^-3.
• Concentration (g dm^-3) = mass of solute (g) / volume of solution (dm³)
• Concentration (mol dm^-3) = number of moles of solute (mol) / volume of solution (dm³)

Relationship between Number of Moles with Molarity and Volume of a Solution

• Molarity (mol dm^-3) = number of moles of solution (mol) / volume of solution (dm³)
• M = molarity of solution (mol dm^-3), V = Volume of solution (dm³), n = Number of moles of solute (mol)
• M = n/V

Preparation of Standard Solutions

• Standard solution = a solution in which its concentration is accurately known.
• Standard solution is prepared by using volumetric flask with a fixed volume (100 cm³, 200 cm³, 250 cm³, 500 cm³ and 1000 cm³)

Preparation of a Solution by Dilution Method

• Dilution = a process of diluting a concentrated solution by adding a solvent (water) to obtain a more diluted solution.
• The concentration of the solution decreases after dilution.
• The number of moles of solute in the solution remains unchanged after dilution.
• (M_aV_a) / 1000 = (M_bV_b) / 1000
• M_a = Initial molarity of solution, M_b = Final molarity of solution, V_a = Initial volume of solution and V_b = Final volume of solution

Relationship between pH Values and Molarities of Acids or Alkalis

• pH values depends on degree of dissociation and molarity / concentration of hydrogen ions, H⁺/ hydroxide ions, OH^- in the solution.
• The higher the molarity of a strong acid, the lower is its pH value.
• The higher the molarity of a strong alkali, the higher is its pH value.
• The molarity of an acid can be changed when (i) water is added, (ii) an acid of different concentration is added and (iii) an alkali is added.