Consider the following reaction. 4Fe + 3O2 2Fe2O3

Question

Consider the following reaction. 4Fe + 3O2 2Fe2O3

How many moles of O2 are required to react completely with 54.3 mg of Fe?

How many grams Fe2O3, are produced from the complete reaction of 32.2 g of Fe?

Answer 1

4 Fe + 3 O₂ -->2 Fe₂O₃

 

The first question involves a mass of Fe, you will need the molar mass of Fe.  [1 mol Fe=55.85 g Fe]  The question asks about the moles of O₂ but starts with information about Fe, you will need the mole ratio from the balanced chemical equation.  [3 mol O₂ = 4 mol Fe]  Because molar mass is grams per mole and the question starts you with mg, you need the conversion between g and mg.

 

? moles O₂ = 54.3 mg Fe x (10^-3 g / 1 mg) x (1 mol Fe/55.85 g) x (3 mol O₂ / 4 mol Fe) 

(54.3 * 10^-3 *1*3)/(1*55.85*4) = 0.1629 / 223.4 = 0.000729 mol O₂

 

How many grams Fe₂O₃, are produced from the complete reaction of 32.2 g of Fe?

 

Question asks about grams of Fe₂O_3; you need the molar mass of iron(III) oxide. [1 mol Fe₂O₃ = 159.70 g]

Question starts with 32.2 g Fe; you need the molar mass of Fe. [1 mol Fe = 55.85 g]

Question involves both iron and iron(III) oxide; you need the mole ratio from the balanced chemical equation.  [4 mol Fe = 2 mol Fe₂O₃]

 

?g Fe₂O₃ = 32.2 g Fe * (1 mol Fe/55.85 g Fe) * (2 mol Fe₂O₃ / 4 mol Fe)* (159.70 g / 1 mol Fe₂O₃)

(32.2*1*2*159.7) / (55.85*4*1) = (10248.68) / (223.4) = 46.0 g Fe₂O₃

Answer 2

Hi Miriel,

This problem is only slightly more complicated than your other problem (KClO₃) 

 

The balanced reaction is 4Fe + 3O₂ = 2Fe₂O₃

 

The reason this is a little more complex is that the mole ratios aren't 1:1 like they were in the other problem.

 

Here are the steps you should take to solve problems like this:

 

1. Balance the equation (you did that already)

2. Convert what you are given in grams into moles. In this case you'll want to convert 54.3 mg of Fe into moles

3. Find the mole ratio of what you're trying to find (in this case O₂) divided by what you are given (in this case Fe).

The mole ratio is simply a fraction using the coefficients from the balanced chemical reaction.

In this case, you want O₂ and are given Fe, thus your mole ratio is 3 O₂ / 4 Fe

4. Multiply your answer from 2 by your mole ratio in 3 to get moles of what you're trying to find

5. Now you solved the number of moles of what you're trying to find, so convert it back to grams!

 

 

Ok so in this problem:

Step 1: done

Step 2: First convert mg to g: 54.3mg Fe = 0.0543g Fe

Now, 0.0543g (1 mol Fe / 55.8g) = 9.73 x 10^-4 moles of Fe

Step 3: We found the mole ratio is 3O₂/4Fe

Step 4: multiply steps 2 and 3 together: 9.73 x 10^-4mol of Fe * 3O₂/4Fe = 7.30 x 10^-4 mol O₂ needed

 

We don't need to do step 5 because the question wants the number of moles of O₂ which is 7.30 x 10^-4

 

 

To solve the second part, you start with 32.2g of Fe

 

Step 1: balance... done

Step 2: Convert to moles:  32.2g Fe (1 mol Fe / 55.8g) = 0.577 mol Fe

Step 3: Mole ratio of Fe₂O₃ to Fe is (2 Fe₂O₃ / 4 Fe) So the mole ratio is 2/4

Step 4: multiply steps 2 and 3: 0.577 * 2/4 = 0.289 mol Fe₂O₃ is formed

Step 5: convert product back to grams: 0.289 mol Fe₂O₃ * ((2*55.8+3*16)grams / 1 mol Fe₂O₃)

0.289 mol Fe₂O₃ * ((2*55.8+3*16)grams / 1 mol Fe₂O₃)

=0.289 * (159.6g) = 46.1 grams of Fe₂O₃ is produced

 

Hope this helped!