Stoichemistry Problems?

Question

Problem #1:

Aluminum oxide is formed from the reaction of metallic aluminum with oxygen gas. How many moles of Aluminum are needed to form 3.4 moles of Aluminum oxide?

Hint: This is a simple ratio problem. Just use the molar ratio from the balanced equation. This was covered in 5.09. Mass is not involved.

Problem #2:

Determine the number of grams of NH3 produced by the reaction of 3.5g of hydrogen gas with sufficient nitrogen gas. (Check for diatomic elements). Use the flowchart below for help.

Given quantity in grams--> change to moles----> multiply by ratio from balanced equation -----> convert wanted quantity from moles to grams.

Answer 1

Problem #1:

Aluminum oxide is formed from the reaction of metallic aluminum with oxygen gas. How many moles of Aluminum are needed to form 3.4 moles of Aluminum oxide?

4 Al  & 3 O2 -->  2 Al2O3

? mol Al = 3.4 mol Al2O3 @ 4 mol Al / 2 mol Al2O3=

your answer: 6.8 mol Al

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Problem #2:

Determine the number of grams of NH3 produced by the reaction of 3.5g of hydrogen gas with sufficient nitrogen gas.

find moles of H2 using the molar mass of H2:

3.5 grams H2 @ 1 mol H2 / 2 grams =  1.75 moles H2

find the moles of NH3 produced, using the ratio in

1 N2  & 3  H2    -->  2 NH3

1.75 mol H2 @ 2 mol NH3  / 3 mol H2 = 1.17 moles of NH3

find the grams of NH3 using its molar mass:

1.17 moles NH3 @ 17 g/mol = 19.8 grams of NH3

if you are watching significant digits, as well...

your answer should be rounded off to: 20. grams of NH3