Question

You have two beakers. One contains a solution of HCl at pH = 1.0. The other contains a solution of NaOH at pH = 13. Into a third beaker, you slowly and cautiously pour 20 mL of the HCl and 20 mL of the NaOH. After complete stirring, the pH of the mixture will be

A) 2.0.
B) 12.0.
C) 7.0.
D) 5.0.
E) 9.0

Answer 1

The final pH will be 7.0 (C)

Step-by-step explanation:

The pH is a scale to measure the concentration of protons in a solution. The followiing equation shows the mathematical definition of pH:

`pH = - log [H+]`

Another important equation is the one correlating the values of pH and pOH, which regards the concentration of hidroxide ions in solution;

`pKw = pH + pOH` , with pKw = 14

Finally, it is necessary to know that when an acidic and an basic solutions are mixed quantitatively, menaing with no excess of hydrogen or hydroxide ions, a neutralization reaction occurs:

HCl + NaOH → NaCl + H₂O

Now, lets calculate the H⁺ and OH⁻ concentrations in both solutions:

HCl: pH = 1.0, which means pH = -log[H⁺] ∴ [H⁺] = 1 x 10⁻¹ mol/L

NaOH: pH = 13.0, which means pOH = 1.0 pOH = -log[OH⁻] ∴ [OH⁻] = 1 x 10⁻¹ mol/L

Since the concentrations of H⁺ and OH⁻ are the same and equal volumes of each solutions were used, a neutralization reaction will happen and the final pH will be equal to the pH of water (pKw) which is 7.0.