If hydrogen was burnt in a chamber full of oxygen, what would be the effect on the chamber pressure and why?

The equation for the reaction is; 2H₂ +O₂ = 2H₂O

From the equation it can be seen that 2 moles of hydrogen reacts with 1 mole of oxygen to produce 2 moles of water vapour. Since the reaction used more moles in reactants (3) than it produced in products (2), the remaining volume of gas expands to fill the space reducing the pressure.