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pH calculation questions » pH of strong acid

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

eq. 7.67.6

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

eq. 7.47.4

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.

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