# Angela



## Angela SDR (Sep 4, 2019)

Please help! I do not understand why the mass of the dish would not be subtracted from the mass of the water in this example??? The mass of the dish should have nothing to do with the Water content however it is not subtracted when the mass of the water is identified as 98 g?


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## DoctorWho-PE (Sep 4, 2019)

Because the wet soil includes the dish, and the dry soil includes the dish.  So the mass of the water would simply be the wet sample (including dish) minus the dry sample (including dish). You do however have to subtract the dish mass from the dry sample to obtain the soil mass.


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## ChebyshevII PE PMP (Sep 4, 2019)

I’m probably the wrong type of engineer to be answering soil questions specifically, but...

If you look at a) and b) you will notice that the mass of the dish is already included in both numbers in the solution. Then when you get to c), the equation is a) - b), which can also be written as c) = (Mws + Mdish) - (Mds + Mdish), where Mws is mass of wet soil, Mds is mass of dry soil, and Mdish is mass of dish. Mdish cancels out of this equation, so you have c) = Mws - Mds, the result of which gives you the water content that evaporated from the original sample.

The solution assumes that measurement of the soil (both wet and dry) was done with the dish, which makes logical sense to me. Not knowing other details about the book, though, it does look like the problem is worded strangely.


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