Acidity vs alkalinity

Pure water ionises to give hydrogen and hydroxyl ions with the concentrations of H+ and OH at 10-7 M each.

H_2O\rightleftharpoons H^++OH^-

This makes the pH of pure water 7. Water also ionises in aqueous solutions (i.e. solutions of compounds, e.g. NaCl, where water is the solvent). We call an aqueous solution of pH 7, a neutral solution. If the pH of an aqueous solution is less than 7, [H+] > [OH] and the solution is termed acidic. If the pH of the aqueous solution is more than 7, [H+] < [OH] and the solution is basic or alkaline.



20.0 ml of 0.0025 M of HCl is added to 50.0 ml of water. What is the pH of the final solution? Why is it not necessary to consider H+ from the dissociation of water in the computation?


No. of moles of H+ in 20 ml of HCl = \frac{0.0025\times 20}{1000}

[H+] in 70 ml of solution = \frac{0.0025\times 20}{1000}/0.070

pH=-log(\frac{0.0025\times 20}{1000}/0.070)=3.15

We do not consider the concentration of H+ from water as it is insignificant. According to Le Chateliers’ principle, the presence of H+ from aqueous HCl causes the equilibrium of the dissociation of water to shift to the left, reducing the concentration of H+ in water to less than 10-7 M. Even if we assume that , the resulting pH value of -log\left \{[(\frac{0.0025\times 20}{1000})+(10^{-7}\times \frac{50}{1000})]/0.070\right \} is still 3.15.

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