problem F13

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katherinemurk 2B
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Joined: Wed Nov 15, 2017 3:02 am

problem F13

Postby katherinemurk 2B » Tue Oct 02, 2018 3:51 pm

Problem F13 in the textbook asks "In an experiment, 4.14g of phosphorus combined with chlorine to produce 27.8g of a white solid compound. What is the empirical formula of the compound?
In the solutions manual it skips the step of finding mass percentage of each element. So instead of dividing the mass of the element by the mass of the sample and then dividing by the molar mass of each element to find the amount in moles, it skips to the part of finding the amount in moles. I dont understand why some problems include that step and others don't?

Gary Qiao 1D
Posts: 66
Joined: Fri Sep 28, 2018 12:26 am

Re: problem F13

Postby Gary Qiao 1D » Tue Oct 02, 2018 4:33 pm

I think the solutions manual skips finding the mass percentage of each element in this problem because the mass percentage, in this case, would only be needed to find the mass of each element and the problem already gives the mass of phosphorus (4.14g) and chlorine (27.8-4.14g). With this, you can look directly for the moles, so the extra step of finding the mass percentage is redundant. In other problems, however, the question may ask you to find the mass percentage of the molecule. Otherwise, the step is usually not needed if the word problem gives the mass of each element. Hope this helps!

Andrea Zheng 1H
Posts: 61
Joined: Fri Sep 28, 2018 12:26 am

Re: problem F13

Postby Andrea Zheng 1H » Wed Oct 03, 2018 1:25 pm

In lecture, Lavelle used the mass composition to provide the mass (g) of an element given a sample of 100 g (ex: given a mass composition of C = 45%, the mass of C would be 45g). Because the problem directly gives you the mass (g), you don't need to calculate the mass composition, as you can already convert the mass (g) into moles using the molar mass of the element (g/mol).

Posts: 57
Joined: Fri Sep 28, 2018 12:18 am

Re: problem F13

Postby daniella_knight1I » Wed Oct 03, 2018 8:52 pm

Some problems may go more in-depth than others but the steps they take in each individual problem regarding this topic is the same. Just make sure you realize the missing steps, i.e. converting to moles by dividing by each molar masses and you should be good. If anything, look at other problems that contain all the steps to really visualize how to do the problems.

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