Assume that we have this reaction 2H3PO4⇒ P2O5 + 3H2O
and when the rate of reaction is the speed at which the reactants can be changed and are converted into products So, according to our balanced reaction:
The rate of reaction = rate of formation of P2O5
and the rate of this reaction also = 0.5 rates of decomposition of H3PO4
∴ the rate of decomposition of H3PO4 = 2 rates of formation of P2O5
So when the rate of formation P2O5 = 2 x 10^-4 M/s
So the rate of decomposition of H3PO4 = 2 x 2 x 10^-4
= 4 x 10^-4 M/s