The Student Room Group

Calculating pH

How do you know that the concentration of H+ ions in this solution is 4.0? What is the chemistry behind the assumption that concentration of HCl and H+ are the same?

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The phrase "assuming that it is fully ionised" suggests that all of the HCl has been converted to hydrogen ions and chlorine ions. Therefore the concentration of hydrogen ions would be 4mol/dm^3
@Export kid so when an acid is fully ionised do we always assume that concentration of the acid and its H+ ions are the same?
Original post by at.midnight
@Export kid so when an acid is fully ionised do we always assume that concentration of the acid and its H+ ions are the same?


Not for all acids. HCl only donates one H+ per molecule so in that case H+ concentration=acid concentration.

In the case of sulphuric acid (where 2 H+ ions are donated per molecule) the H+ concentration would be twice that of the acid concentration

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