Usually, concentration of one of the substances in solution is determined by titration with a substance of known concentration, that reacts with it cleanly.
When they react in 1:1 ratio, the relationship between their concentration (in molarity) and volume is:
`V_1S_1=V_2S_2` --- (i)
Out of the four parameters f the reltion, any unknown parameter can be evaluted if rmaining three are known.
But as exemplified in the given reaction, when they react in 1:2 ratio, the relationship between their concentration (in molarity) and volume takes the form:
`2V_AS_A=V_BS_B` --- (ii)
Thus, if three of the parameters are known, the unknown one can be determined.
In the given example, two are given. So, a third parameter has to be determined first through titration, only then the relationship (ii) would yield the fourth parameter.