Exploring the De Broglie Wave and Momentum in a Trapped Particle's Excited State

[Apashanka]

For a particle trapped in a region of length L the de broglie wave for the 1st excited state is a pure sine wave from 0 to 2pi
for which the particle momentum can be calculated as 2h/L from de broglie relation
Whereas from energy quantisation relation p=nh/2L where n is the state integer,for n=2 ,p=h/L
Why does so happen?

 

[DrClaude]

Whereas from energy quantisation relation p=nh/2L where n is the state integer,for n=2 ,p=h/L

Particle-in-a-box eigenstates are not eigenstates of the momentum operator, therefore you can't say that it corresponds to a given value of momentum.