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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: artem.metelskiy on March 31, 2011, 06:54:51 PM

Title: Calculating moles and mass problem
Post by: artem.metelskiy on March 31, 2011, 06:54:51 PM

I need some help to solve this problem:
Iron reacts slowly with oxygen and water to form a compound commonly called rust (Fe₂O₃ * 4H₂O) For 45.2 kg of rust. Calculate: a) the moles of compound; b) The moles of Fe₂O₃; c) the grams of Iron.

I'm not sure where to start, should I frist balance the chemical equation Fe+O₂+H₂O -> Fe₂O₃ * 4H₂O?

Title: Re: Calculating moles and mass problem
Post by: DevaDevil on March 31, 2011, 09:48:29 PM

Balancing is always a good thing to do.

for the number of moles you first will need to calculate the molar mass of rust.

for the number of grams of iron you will need your balanced equation.

Title: Re: Calculating moles and mass problem
Post by: opti384 on April 01, 2011, 02:06:45 AM

When you have a chemical equation, you should try to always balance it and check it if it's balanced.

Title: Re: Calculating moles and mass problem
Post by: artem.metelskiy on April 01, 2011, 02:26:51 AM

for part a) Molar mass of rust is 159.7 so 4520g/159.7=28.30


for the rest of problem I don't know how to balance this equation when it has Fe₂O₃ * 4H₂O part in it. Can someone please explain? Thanks.

Title: Re: Calculating moles and mass problem
Post by: AWK on April 01, 2011, 02:49:37 AM

45.2 kg = 45200 g
Molar mass of rust is 157.9 + 4x18
Your calculations are based on formula (of hydrate) and there is no need for using balancing of reaction (though it may be helpful in this way)
Fe₂O₃^.4H₂O= Fe₂O₃ or (2Fe + 3/2O₂) + 4H₂O

Title: Re: Calculating moles and mass problem
Post by: artem.metelskiy on April 01, 2011, 03:38:32 AM

Quote from: AWK on April 01, 2011, 02:49:37 AM

Fe₂O₃^.4H₂O= Fe₂O₃ or (2Fe + 3/2O₂) + 4H₂O

Since we don't want to have 1/2 coefficient in front of O₂, should I multiply the whole equation by 2?
4Fe+3O₂+8H₂O->2Fe₂O₃*8H₂O
Would that be a correct balanced equation?

Title: Re: Calculating moles and mass problem
Post by: AWK on April 01, 2011, 04:36:24 AM

2(Fe₂O₃*4H₂O) is much better