Mass question - please dumb it down for me

[lorka150]

I am studying for a test and this is a question I got incorrect on an assignment. They give us the correct answer, but since we only hand in our answers (and not the work shown), I don't know how they got it.

I wonder if someone could, in the 'dumbest' way possible, explain this to me. Chemistry is not my forte.

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An ore contains 42.9% of the mineral CaF2, which is a source of the elemtn F. How much ore must be processed in order to obtain 42.0 kg of F?
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The answer (the correct one) is 201 kg.



Please help! Thank you.

 

[mrjeffy321]

Even though your answer was wrong, what method did you use to approach this problem?

I would work this problem in the following way,
-How many moles of F2 (since Flourine is a diatomic gas) are in 42.0 kg?
-How many moles of CaF2 are required to produce ___ moles of F2?
-What is the mass of ___ moles of CaF2?
-If your CaF2 sourse is is only 42.9% pure by weight, how much you need?

 

[lorka150]

Thank you for your reply. I tried it and honestly, just don't know what you're talking about. Thank you anyway.

 

[mrjeffy321]

What part(s) don't you understand?

Do you understand the need to convert 42.0 kg of F2 into moles of F2?
I think this would be the most obvious step.

Once you know the number of moles of F2 you need, you can calculate the number of moles of CaF2 needed by using the molar ratio of the number of moles of CaF2 reacted to moles of F2 produced...assume the chemical reaction is,
CaF2 --> Ca + F2

One you know how much CaF2 you need you cand find the mass of the CaF2 using the molar mass. This would tell you how much would be needed if your sample was 100% CaF2.
Since it is not 100% CaF2 you need to find a relationship between the percentage of CaF2 contained within the sample to the total mass needed in order to get the previously calculuted amount of CaF2.