[sjb]

In origin Lithium was isolated in ampoule so I agree with you that initial pressure was 0.

About number of moles - if from 1 mole Lithium we get 1 mole Trithium and 1 mole Helium - doesn't it mean that generally we have 2 moles of ideal gas? Why not, explain please.

What is the formula of tritium gas?

[beruniy]

T₂
I understand. I must transfer nuclear reaction into chemical:

2Li + 2n = T₂ + 2He

number of moles
n(He) = n(Li) = 0.002 mole; n(T₂) = 0.001 mole
generally, n = 0.003 mole.