General information:
Salicylic acid is a weak monoprotic acid. It partially ionizes (dissociates) in solution to produce hydronium and salicylate ions. It has a molar mass (molecular weight) equal to 138.12 g/ mole. It has Ka value equal to 1.06 × 10-3 at 25 ºC.
Molar concentration of a weak monoprotic acid required to produce certain pH value in solution can be calculated from the following equations (see where it came from):
Where C is molar concentration of the monoprotic acid, [H+] is molar concentration of hydrogen (hydronium) ion, and Ka is the value of the acid ionization constant.
Calculations:
Mass of salicylic acid needed to make 500 ml solution with a concentration of 0.00578 mole/ liter is calculated as the following:
Volume of solution in liters = 500 ml × (0.001 liter/ ml) = 0.5 liter
Number of moles = concentration × volume = (0.00578 mole/ liter) × 0.5 liter = 0.00289 mole
Mass of salicylic acid = number of moles × molar mass = 0.00289 mole × (138.12 g/ mole) = 0.40 g
To make the preparation, dissolve 0.40 g salicylic acid in freshly distilled water to make 500 ml solution (using 500 ml volumetric flask).
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Salicylic acid is a weak monoprotic acid. It partially ionizes (dissociates) in solution to produce hydronium and salicylate ions. It has a molar mass (molecular weight) equal to 138.12 g/ mole. It has Ka value equal to 1.06 × 10-3 at 25 ºC.
Molar concentration of a weak monoprotic acid required to produce certain pH value in solution can be calculated from the following equations (see where it came from):
[H3O+] = [H+]
C = ((2 × [H+] + Ka)2 - Ka2 ) ÷ (4 × Ka)
C = ((2 × [H+] + Ka)2 - Ka2 ) ÷ (4 × Ka)
Where C is molar concentration of the monoprotic acid, [H+] is molar concentration of hydrogen (hydronium) ion, and Ka is the value of the acid ionization constant.
Calculations:
pH = - log [H+]
[H+] = 10-pH
[H+] = 10-pH = 10-2.7 = 0.0020 mole/ liter
C = ((2 × 0.0020 + (1.06 × 10-3))2 - (1.06 × 10-3)2 ) ÷ (4 × (1.06 × 10-3))
C = ((0.00506)2 - (1.12 × 10-6 )) ÷ (0.00424)
C = ((2.56 × 10-5) - (1.12 × 10-6 )) ÷ (0.00424)
C= (2.45 × 10-5) ÷ (0.00424)
C = 0.00578 mole/ liter
Use online pH calculator to check that 0.00578 mole/ liter salicylic acid produces solution pH = 2.7.
Mass of salicylic acid needed to make 500 ml solution with a concentration of 0.00578 mole/ liter is calculated as the following:
Volume of solution in liters = 500 ml × (0.001 liter/ ml) = 0.5 liter
Number of moles = concentration × volume = (0.00578 mole/ liter) × 0.5 liter = 0.00289 mole
Mass of salicylic acid = number of moles × molar mass = 0.00289 mole × (138.12 g/ mole) = 0.40 g
To make the preparation, dissolve 0.40 g salicylic acid in freshly distilled water to make 500 ml solution (using 500 ml volumetric flask).
Related posts:
Prepare 500 ml acetic acid solution with a pH value equal to 2.7.
Calculate mass of hydrochloric acid (HCl) needed to prepare 100 ml solution with pH equal to 1.5
Calculate pH of 100 ml solution containing 0.05 g acetic acid and 0.06 g sodium acetate.
Calculate pH of 1 liter solution containing 0.5 g benzoic acid.
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