Question

9. A titration is carried out for 25.00mL of 0.100M HCl (strong acid) with 0.100M of a strong base NaOH. Calculate the pH after the following volumes of NaOH are added: a) 0.00mL b) 12.50mL c) 25.00mL d) 37.50mL

Answer 1

The titration of a strong acid with a strong base results in a salt that does not hydrolyze the aqueous media it is in and remains neutral, without contributing to any change in pH. So the pH of the solutions at various volumes of NaOH added is only due to the protons or hydroxide ions in the medium. Since HCl and NaOH react in a 1:1 molar ratio, the moles of NaOH required to neutralize the HCl completely will be equal to the moles of HCl present.

From the equilibrium constant for the autolysis of water, Kw = 1*10^-14, which gives pH + pOH = 14.

pH = -log[H⁺] and pOH = -log[OH^-]

a) When 0 mL of NaOH is added, the pH is only due to the HCl in the solution. So, pH = -log[0.1] = 1.

b) 12.5 mL of 0.1 M NaOH contains 0.1*(12.5/1000) since molarity is defined as the moles of a substance per liter of solution. This gives 0.00125 moles. Similarly, 25 mL of 0.1 M HCl solution will contain 0.1*(25/1000) = 0.0025 moles.

Since 0.00125 moles of HCl will be neutralized by that many moles of NaOH, the concentration of HCl will now be 0.00125*(1000/(25+12.5)) = 0.0333 M. The pH will therefore be -log[0.0333] = 1.4775.

c) When 25 mL of NaOH is added, it will completely neutralize all of the HCl present in the solution since the moles of NaOH will then be equal to that of HCl. As mentioned before, NaCl, the product of this neutralization reaction will not contribute to the pH. So the pH of this solution will be 7.

d) 37.5 mL of 0.1 M NaOH will contain 0.1*(37.5/1000) = 0.00375 moles of NaOH. Since there is only 0.0025 mole of HCl, the excess hydroxide ion concentration will be 0.00125 mol. The total volume of the solution will be 25+37.5 mL = 62.5 mL.

So, the concentration of hydroxide ions will be 0.00125*(1000/62.5) = 0.02. The pOH will therefore be -log[0.02] = 1.6989, giving the pH of the solution to be 14 - 1.6989 = 12.3011.