• # question_answer When is the law of definite proportions not obeyed?

The law of definite proportions is not obeyed when the element exists in isotopic forms. For example, in $HCl$ the elements $H$ and $Cl$ may be in the ratio of 1 : 35 and 1 : 37 by mass in case $^{35}Cl$  and $^{37}Cl$ isotopes are considered.