Addition of Acids (H2SO4 and HCl)
Problem 1
What is the pH of a 1.2 mM H2SO4 solution at 25?
Answer
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 |
Note. Sulfuric acid is a diprotic acid that donates two H+. The first dissociation step is complete (strong acid) while 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. In fact, both values add up to 1.2 mM, which is the input concentration of H2SO4.
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?
Answer
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 final result is shown in the right screenshot: The addition of 0.2 mM HCl decreases the pH from 8.20 to 7.24.