Neglecting the contribution of hydronium ions from the autoprotolysis of water (approximation), it can be said that each hydronium ion in the solution comes from a strong acid molecule, therefore the number of moles of acid (per liter) prior to the dissociation is equal to the number of moles (per liter) of hydronium ion after dissociation: $c_{HB}$ $=$ $[H_3O^+]$
Strong acid: $pH$ $=$ $-log\;c_{HB}$
Example: The $pH$ of a $0.2\;M$ solution of nitric acid is: $pH$ $=$ $-log0.2$ $=$ $1.7$