What mass of O2 will react with exactly 6.00 g P to form P2O5 in the following reaction? 4P + 5O2 → 2P2O5

Question

similar to stoichiometry

Answer 1

This is a classic stoichiometry problem! We'll break it down into several smaller steps:

1. Mass of P to moles of P using molar mass of P
2. Moles of P to moles of O₂ using balanced reaction
3. Moles of O₂ to grams of O₂ using molar mass of O₂
4. Significant Figures

1. Mass of P to moles of P using molar mass of P
2. The initial quantity is grams of P, which is convenient for a laboratory setting — we'd use a scale to measure the mass of P, and finding the number of moles directly isn't feasible. We can use the molar mass of P to convert g of P into moles of P, which will be handy in step 2:
3. 6.00 g P * (1 mol P / 30.9738 g P) = 0.193712 mol P. Note: we used moles over grams to cancel any dependence on grams and result in moles.
4. Moles of P to moles of O₂ using balanced reaction
5. Next, we'll convert from moles of P to moles of O₂ because we're aiming for information about the O₂ — namely the mass of O₂ — and we want to cancel anything having to do with P. We'll use the molar ratios from the balanced chemical reaction, which tells us how many moles of O₂ and P take part in the reaction:
6. 0.193712 mol P * 5 mol O₂ / 4 mol P = 0.24214 mol O₂
7. Moles of O₂ to grams of O₂ using molar mass of O₂
8. Lastly, we convert moles of O₂ into grams of O₂ (the unit and species of interest) using molar mass:
9. 0.24214 mol O₂ * 15.999 g O₂ / 1 mol O₂ = 3.87399 g O₂. Note: we used grams over moles to cancel any dependence on moles and result in grams.
10. Significant Figures
11. This process began with a measurement with three significant figures (0s following a decimal count). The conversions from steps 1-3 all used multiplication/division, so our final answer should preserve three sig figs:
12. 3.87399 g O₂ → 3.87 g O₂