1. I am stuck on this chemistry problem using Mastering Chemistry to do my homework. This is the problem. When I enter the answers it keeps saying is that "One or more of the units you entered is not recognized."

For part A my answer was 426 g P2O5

and for part B 369 g P2O5

**Problem**

Consider a situation in which 186 g of P4 are exposed to 208 g of O2.

P4+5O2→2P2O5

PART A) What is the maximum amount in moles of P2O5 that can theoretically be made from 186 g of P4 and excess oxygen?

PART B) What is the maximum amount in moles of P2O5 that can theoretically be made from 208 g of O2 and excess phosphorus?

2. Hi Teh!

The problem is asking for the number of moles.
So the answers shouldn't be in grams.
Moreover, the unit should be left out.

Note that to find moles from grams we have to divide by the atomic mass in atomic mass units.
So for instance 32 g of $O_2$ is 1 mole of $O_2$.

For part $(A)$ we have 186 g phosporus and $P$ has an atomic mass of 31 amu, or 31 g/mol.
That makes:
$$\frac{186}{31} = 6.0 \text{ mol of P atoms}$$
The reaction is:
$$P_4 + 5O_2 \to 2P_2O_5$$
The resulting product has $P_2$ in it, meaning 2 $P$ atoms per molecule, so there will be:
$$\frac {6.0} 2 = 3.0 \text{ mol of P_2O_5 molecules}$$

So the requested number of moles is $3.0$.