What is pH of 0.0002M HI solution?

What will be pH if 1 mL of the above was added to 999 mL of pure water?

HI is a strong acid so we can assume it is fully dissociated.

3×10^{-4} M solution is concentrated enough to allow us to neglect water dissociation (see table in strong acid section). Thus we can use 7.6 equation

and the pH calculated is **-log(3×10 ^{-4})=3.70**.

After the solution has been diluted 1000 times HI concentration is 2×10^{-7} M and we are in the area where water dissociation may spoil results of the 7.6 equation. Thus it wil be better to use the equation 7.4

and calculated **pH=6.62** (you know how to solve quadratic equation, don't you?).

Using equation 7.6 we will get pH=6.70. Difference is not large - but clearly visible.

As additional confirmation our pH calculator shows exactly the same results - 3.70 and 6.62.