Can someone balance my equation?

Question

I can't figure out how to balance this one. Does anyone know?
H3AsO4------> AsH3 + H20

Thanks

Answer 1

H3AsO4 --> AsH3 + H2O

Right now, you have 3 H on the left and 5 H on the right, 1 As on each side, and 4 O on the left and 1 O on the right. Start by adding a coefficient of 4 for H2O on the right, so you'll have 4 O on both sides.

H3AsO4 --> AsH3 + 4H2O

Now we have 7 H on the right. H is difficult to balance because it occurs in both products. This approach is proving quite challenging, so it might be better to treat it algebraically. Starting over, I'll use variables as coefficients.

XH3AsO4 --> YAsH3 + ZH2O

This lets me write three equations.

H: 3X = 3Y + 2Z
As: X = Y
O: 4X = Z

Now, this is a problem. If X = Y, then the first equation becomes 3X = 3X + 2Z => 2Z = 0 => Z = 0. But we can't have a coefficient equal to zero, so the original chemical equation must be wrong. Note that I already assumed it was wrong, in that H20 was really meant to be H2O, because otherwise the oxygen from the reactant would have completely disappeared.