For school, I am doing a small study on a chemical reaction involving 2 M Sulfuric Acid (H₂SO₄) and Zinc (Zn). I have done some background research on the process to calculate the pH of a solution, but the expected value doesn't seem to match either the initial pH of the H₂SO₄, or the change in pH over time.
For reference, we calibrated our pH probe multiple times to an accuracy within ± 0.1 of the expected values of the calibration solutions (pH 4 and pH 7). Water displayed a pH of 7.06.
According to the probe, the pH of 2 molar H₂SO₄ is ≈ -0.25 ± 0.03
, which I do not understand. Research indicates that the pH of a solution can be calculated using $-log_{10}(\ce{H^+})$. When completely dissociated in water, 2 molar sulfuric acid should have a H⁺
value of 4, so the pH should be equal to $-log_{10}(4)$, which is ≈ -0.602
, over 0.35 off from the measured value of -0.25
.
Also, as the reaction between Zinc and Sulfuric Acid progressed, the pH of the solution decreased (from ≈ -0.25
to ≈ -0.30
). This also makes no sense to me, as my intuition would tell me that the pH would increase, not decrease. This is the main result I am seeking answers for.
If anyone could explain this to me, it would be incredibly helpful for my study.