```
pH = -log[H3O+]
```
where [H3O+] represents the concentration of hydronium ions in moles per liter (M).
In this case, we are given the concentration of H3O+ as 4.3 x 10-8 M. Plugging this value into the formula, we get:
```
pH = -log(4.3 x 10-8)
```
```
pH ≈ 7.37
```
Therefore, the pH of a solution with a H3O+ concentration of 4.3 x 10-8 M is approximately 7.37. This indicates that the solution is slightly basic, as a neutral solution has a pH of 7 and any value above 7 is considered basic.
