Question about percentage purity and percentage yield [closed]








**Closed.** This question is [off-topic](/help/closed-questions). It is not currently accepting answers.
 
 






---








 **Homework questions** must demonstrate some effort to understand the underlying concepts. For help asking a good homework question, see: [How do I ask homework questions on Chemistry Stack Exchange?](https://chemistry.meta.stackexchange.com/questions/141/how-do-i-ask-homework-questions-on-chemistry-stack-exchange)




Closed last month.








[Improve this question](/posts/175137/edit) 



While doing questions on percentage yield, this one took my attention


As a result of the reaction of 20 grams of hematite with a purity of 60% with a sufficient amount of pure aluminum, how much impure iron with a purity of 70% will we get? (The percentage yield of this reaction is 40%)


$$\ce{2 Al + Fe2O3 -> 2 Fe + Al2O3}$$


My question is that why does it say "How much impure iron will we get " aren't all products of a reaction pure ?




To start let's assume that you're using some reaction to make some compound in the lab. There are two "products." First the products as given in the the chemical formula for the reaction and second there are the products that you collect. The products that you collect are never *absolutely* pure. 99.99% pure would be essentially pure since working to 4 significant figures in a typical chemistry lab is very good work. But that isn't absolutely pure.


Now to discuss the problem you posed. This is just a made up problem and to worry to much about the details will send you down the rabbit hole.


But let's think about the problem this way. Suppose you are going to do the reaction in a crucible and after the reaction is complete you crack the crucible with a hammer to get the "button" of metal as the "collected product."


Now the stated problem throws in some wrinkles to see if you understand chemistry and are not just solving a math problem.


* First the problem states "the reaction of 20 grams of hematite" but then the problem backpedals and states that the hematite is only 60% pure. Who knows what the other 40% is. Make the assumption that the 40% is inert. (The problem would have been better stated as *the reaction of 20 grams of hematite **ore** which is 60% pure.*)
* The problem states that there is "a sufficient amount of pure aluminum" which means that there is at least enough aluminum to react with all the hematite.
* Now, as in my contrived experiment, the problem states that the button of metal produced is only 70% iron. Let's assume that 12.0 grams of hematite should yield *x* grams of pure iron. Then the button would weigh *x*/0.70 grams since it is impure.


 0