Popoff's rule states that during oxidation of ketones, keto group always stays with the smaller alkyl group. For example,

CH3CH2COCH2CH2CH3 on oxidation with HNO3 gives two moles of propanoic acid, where in one mole of propanoic acid the cooh group comes from CH2 of propyl part of ketone.

CH3COCH2CH2CH3 on oxidation gives CH3COOH & CH3CH2COOH, here also the CH2 of propyl group is oxidised.

In both the examples the C=O remains with the smaller alkyl group,viz. CH3CH2 & CH3 respectively.

Mechanism is uncertain but probably proceeds with enol formation and depends on stability of enol