if 0.75 mole of helium gas occupies a volume of 1.5. what volume will 1.2 mole helium occupy at the same teperature and pressure?

Answer 1

1.5 what?
You can work this a couple of ways.
Use PV = nRT and solve for P and plug this back into PV = nRT with n = 1.2, P from above and all others being the same.

A second way is to use ratio/proportion.
(v1/n1) = (v2/n2)

A third way is to use PV = nRT but modify that so that V = nRT/P. Since R,T, & P are constants we can write that as V = nk. Plug in 1.5 and n and solve for k, then redo V = nK and plug in the k you found and the new n and solve for the new v.

Finally, since volume is proportional to mols, it is just 1.5 x 1.2/0.75 = ?

Four ways, take your pick.

Answer 2

0.6