If 0.40 mol of h₂ and 0.15 mol of o₂ were to react as completely as possible to produce h₂o, what mass of reactant would remain?

0.1 mole of H2, that in grams will represent: 1 mol = 2 grams H2,

0.1 moles = 0.2 grams of H2

Explanation:

The complete reaction will it be:

2 H2 + O2  →  2  H2O

1.- That means that for 1 mole of O2 we produce 2 moles of H2O but we just have 0.15 moles of O2 so if we apply a rule of three:

moles of H2O to produce = (0.15  x 2 )/1 = 0.3 moles produced of H2O

2.- So as we can observe in the chemical reaction, you need 2 moles of H2 to produce 2 moles of H2O but you can just produce 0.3 moles, so again if you do a rule of three:

moles of H2 that are going to be used: (0.3 x 2) / 2 = 0.3 moles of H2

3.- Since you have 0.4 moles, but you are just going to use 0.3 , you left 0.1 moles

fire could start, buildings collapsing, and ash falling from the sky.

explanation:

this is because the buildings or plants could catch on fire and the buildings would most likely collapse. lastly ash would fall because all the burning would cause ash to fall from the sky.

hope this .

the answer is 1 i believe