Homework help. I keep getting it wrong. Please give answers

Question

Given the following equation:

4 NH₃ (g) + 5 O₂ (g) →→ 4 NO (g) + 6 H₂O (l)

How many moles of NH₃ is required to produce 35.9 grams of H₂O? 

Answer 1

First make sure you have a balanced chemical reaction equation. And you do, 4 N's on each side, 12 H's on each side, 10 O's on each side, So now you are ready to begin.

The chemical reaction equation is given in units of moles so to use it, you must convert 35.9 grams of H₂O into moles. To do that, you need the molar mass. Use the periodic table to find the atomic mass of H and O and combine as follows:

Element Atomic Mass Number of Atoms Total

H 1.008 2 2.016

O 15.999 1 15.999

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Molar Mass of H₂O: 18.015 g/mol

Divide 35.9 grams by 18.105 g/mol to get 1.993 moles.

No look at the chemical reaction equation. For every 6 moles of H₂O produced it requires 4 moles of NH₃ so use that ratio to convert moles of H₂O into moles of NH₃:

Notice that to get the answer you set up the ratio of 4 moles NH₃ to 6 moles H₂O in the way necessary to cancel out the "moles of H₂O" leaving you with units of "moles of NH₃". Then you just multiply 1.993 by 4 and divide by 6 to get the answer.

Since you started with 3 sig figs, round your answer to 3 sig figs or 1.33 moles of NH₃