Strong acids are those that dissociate completely into their ions. So the dissociation equation for a strong acid is:
HA → H⁺ + A⁻
It is visible from the equation that the number of moles of hydrogen ions released is equivalent to the number of moles of acid. For a given volume,
[HA] → [H]⁺ + [A]⁻
Thus, the assumption is logical and fairly accurate
