HW L35 Question: Using Mole Ratios to predict mass of product

Sofia Lucido 3L
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Joined: Wed Sep 30, 2020 9:51 pm
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HW L35 Question: Using Mole Ratios to predict mass of product

I need help with L35 on the fundamentals homework. I was wondering if someone could go through the steps I took to help me find my error because I keep getting the wrong answer. I have checked all my calculations but I am still not sure if it is a math error or a conceptual error. Thank you!

The question is:

Sodium bromide, NaBr, which is used to produce AgBr for use in photographic film, can itself be prepared as follows (I have balanced it and fixed the error in the third chemical equation):

$Fe+Br_{2}\rightarrow FeBr_{2}$

$3FeBr_{2}+Br_{2}\rightarrow Fe_{3}Br_{8}$

$Fe_{3}Br_{8} + 4Na_{2}CO_{3}\rightarrow 8NaBr + 4CO_{2} + Fe_{3}O_{4}$

What mass of iron, in kilograms, is needed to produce 2.50 t of NaBr?

1. First I converted 2.5 tons of NaBr to $2.50 \cdot 10^{6}$ grams.
2. Then I converted the amount of NaBr in grams to moles using the molar mass. I got 22, 042. 102 mol of NaBr.
3. Next I used the ratio of 8 moles of NaBr : 1 mole of Fe to figure out how many moles of Fe would be produced. I got 2, 755.263 moles of Fe.
4. Then I converted moles of Fe to grams of Fe using the molar mass and got 153, 876.6454 grams of Fe.
5. Last I converted grams of Fe to kg of Fe and got 153. 868 kg of Fe.

The answer in the back of the book is 509 kg of Fe. I'm not sure what I am doing wrong. Thank you for the help!

Crystal Pan 2G
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Joined: Wed Sep 30, 2020 10:04 pm
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Re: HW L35 Question: Using Mole Ratios to predict mass of product

I started by balancing the equations which I see you already did. Then you take 2.50 t and multiply by 1000 kg/1t and 1000 g/1kg to get the mass in grams. Next you multiply by 1 mol NaBr/102.9 g NaBr (so divide by the molar mass of NaBr to get the moles of NaBr) and then you multiply 1 mol Fe3Br8/8 mol NaBr (you use the stoichoimetric coefficients from your balanced equation to get this) and then multiply by 3 mol FeBr2/1 mol Fe3Br8 (also get this from your balanced equations) and then multiply by 1 mol Fe/1 mol FeBr2 (to get the mols of Fe in your compound since the question relates to Fe) and then you multiply by the molar mass of Fe which is 55.84 g Fe/1 mol Fe and you get 5.09 x 10^5 g Fe or 509 kg Fe. Hope that helped

Sofia Lucido 3L
Posts: 100
Joined: Wed Sep 30, 2020 9:51 pm
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Re: HW L35 Question: Using Mole Ratios to predict mass of product

Yes it is very helpful, now I see that I have to use the other mole ratios as well to get to Fe. Thank you for the clarification!