Find the pH at which the hydrogen ion H+ concentration is 6.38×10^-6.

To find the pH, we can use the formula:

pH = -log[H+]

Given that the hydrogen ion concentration [H+] is 6.38×10^-6, we can plug this value into the formula:

pH = -log(6.38×10^-6)
pH = -log(6.38) + log(10^-6)
pH = -log(6.38) - 6
pH ≈ -(-5.1959) - 6
pH ≈ 5.1959 - 6
pH ≈ -0.8041

Therefore, the pH at which the hydrogen ion concentration is 6.38×10^-6 is approximately -0.8041.

The pH of a solution is a measure of its acidity or alkalinity, and is defined as the negative logarithm of the hydrogen ion concentration, [H+]. The pH scale ranges from 0 to 14, with 0 being the most acidic and 14 being the most alkaline (or basic).

In this case, the hydrogen ion concentration is given as 6.38×10^-6, so the pH can be calculated as follows:

```
pH = -log[H+]
pH = -log(6.38×10^-6)
pH = 5.19
```

Therefore, the pH at which the hydrogen ion concentration is 6.38×10^-6 is 5.19.