Final answer:
The ion present in relatively large concentration in the acidic rain sample with a pH of 3.5 is the hydronium ion (H3O+), which is characteristic of an acidified solution such as acid rain.
Step-by-step explanation:
A sample of rainwater with a pH of 3.5 is indicative of an acidic solution. The pH level measures the concentration of hydronium ions (H3O+) present in the water. Since pH is defined as the negative logarithm of the hydronium ion concentration (pH = -log[H3O+]), a low pH value means a high concentration of hydronium ions. Therefore, the ion to be present in relatively large concentration in this rain sample is H3O+.
Acid rain typically contains higher levels of hydronium ions due to the dissolution of acidic oxides like sulfur dioxide (SO2) and sulfur trioxide (SO3), which can form sulfuric acid (H2SO4). The dissociation of sulfuric acid in water leads to an increase in H3O+ ions:
H2O(l) + SO3(g) → H2SO4 (aq)
H2SO4 (aq) → 2H+ (aq) + SO4^2- (aq)
The large concentration of hydronium ions is responsible for the acidic pH of this rainwater sample.