Addition of Acids (H2SO4 and HCl)

Problem 1

What is the pH of a 1.2 mM H2SO4 solution at 25?


pH of a 1.2 mM H2SO4 solution

Switch to pure water (button H2O), then click on Reac, select the reactant “H2SO4” and enter the value 1.2 mmol/L.

Run the calculation with Start. The program outputs the result (see right screenshot):

pH = 2.68

Sulfate and Hydrogen Sulfate. Sulfuric acid is a diprotic acid that donates two H+ ions. The first dissociation step is complete (strong acid) so that no undissociated H2SO4 remains in the solution. The second dissociation step is incomplete leaving two ions in the solution:

hydrogen sulfate HSO4- 0.174 mM
sulfate SO4-2 1.026 mM

You find these concentrations (i.e. the equilibrium speciation) in aqion’s output table Ions. Both values add up to 1.2 mM, which is the input concentration of the sulfuric acid.

More about di- and polyprotic acids is presented here.

Problem 2

Given is a water sample with pH 8.2 (“calcite-2”) which belongs to aqion’s example files. What is the pH after addition of 0.2 mM HCl?


pH of a sample water after addition of 0.2 mM HCl

Click on Open to import the example water “calcite-2.sol”. Then click on Reac and enter for the reactant “HCl” the value 0.2 mmol/L. Run the calculation with Start.

The result is shown in the right screenshot: The addition of 0.2 mM HCl decreases the pH from 8.20 to 7.24.

[last modified: 2023-11-16]