Question

If a 0.050g sample of the boron hydride burns completely in `O_2`, what will be the pressure of the gaseous water In a 4.25-L flask at 30.0°C?

Answer 1

I assume that you mean diborane, `B_2H_6`.
`B_2H_6(g) + 3O_2 rarr B_2O_3(s) + 3H_2O(g)uarr`

Explanation:

You are presumed to have `(0.050*g)/(27.67*g*mol^-1) = ?? mol` diborane.

By the stoichiometry of the reaction listed, for each mole of diborane that combusts, `3*mol` of water are produced.

Assuming ideality, `P` `=` `(nRT)/V` `=` `3xx((0.050*g)/(27.67*g*mol^-1))(xx0.0821*L*atm*K^-1mol^-1xx303K)xx(1/(4.25*L))` `=` `?? atm`.

Why did I use `303*K` instead of the given temperature in Centigrade? Would it make a difference if I used the formula `BH_3` instead of that of diborane?

Please note that this answer rests on the assumption that you mean diborane. There are many other borohydrides.