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Author Topic: Tough questions {moles}  (Read 2017 times)

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aliciaf

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Tough questions {moles}
« on: October 12, 2012, 07:44:50 AM »

1) "1 mole of sand grains would cover the state of Texas to several feet. Estimate how many feet by assuming that the sand grains are roughly cube-shaped, each one with an edge length of 0.10  . Texas has a land area of 268,601 square miles."
     I repeatedly got  a large number of ft, and it doesn't make sense to me at all. This is what I did:
.001mm^3*(Avogadro's number)=6.022*10^20 mm^3
1ft^3/304.8^3 = x/6.022*10^20 mm^3, where x=2.12664923*10^13 ft^3
the square root of 2.12664923*10^13 is 4611560.723, but that still doesn't make any sense.


2) "Lead metal can be extracted from a mineral called galena, which contains 86.6  lead by mass. A particular ore contains 68.5  galena by mass. If the lead can be extracted with 92.5  efficiency, what mass of ore is required to make a lead sphere with a 4.50  radius? Express your answer with the appropriate units."
     I ended up (after a bunch of conversions) with 666.8495375 g of ore. Did I do it right?


3) "Mercury is often used as an expansion medium in a thermometer. The mercury sits in a bulb on the bottom of the thermometer and rises up a thin capillary as the temperature rises. Suppose a mercury thermometer contains 3.450  of mercury and has a capillary that is 0.200  in diameter. How far does the mercury rise in the capillary when the temperature changes from 0.0   to 25.0  ? The density of mercury at these temperatures is 13.596   and 13.534  , respectively.
Express your answer with the appropriate units."
     This one I couldn't even figure out how to approach. I went back to it after the test was over and still couldn't figure it out.

I know it's a lot of work, but I would greatly appreciate any help on any of these questions. I'd like to be able to know how to approach and do them correctly. :-\
« Last Edit: October 13, 2012, 02:12:10 AM by Arkcon »
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DrCMS

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Re: Tough questions
« Reply #1 on: October 12, 2012, 08:33:54 AM »

How can anyone answer your questions when you leave out most of the units?
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aliciaf

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Re: Tough questions
« Reply #2 on: October 12, 2012, 12:53:52 PM »

How can anyone answer your questions when you leave out most of the units?

Sigh. I'm sorry, when I pasted the questions it left out the units for some reason. Nothing has been working for me today. Thank you for pointing it out.

FIXED VERSION:
1) "1 mole of sand grains would cover the state of Texas to several feet. Estimate how many feet by assuming that the sand grains are roughly cube-shaped, each one with an edge length of 0.10 mm. Texas has a land area of 268,601 square miles."
     I repeatedly got  a large number of ft, and it doesn't make sense to me at all. This is what I did:
.001mm^3*(Avogadro's number)=6.022*10^20 mm^3
1ft^3/304.8^3 = x/6.022*10^20 mm^3, where x=2.12664923*10^13 ft^3
the square root of 2.12664923*10^13 ft^3 is 4611560.723 ft, but that still doesn't make any sense.

2) "Lead metal can be extracted from a mineral called galena, which contains 86.6 % lead by mass. A particular ore contains 68.5 % galena by mass. If the lead can be extracted with 92.5 % efficiency, what mass of ore is required to make a lead sphere with a 4.50 cm radius? Express your answer with the appropriate units."
     I ended up (after a bunch of conversions) with 666.8495375 g of ore(using 100 g of galena and ore to start with to make calculations easier). Did I do it right?


3) "Mercury is often used as an expansion medium in a thermometer. The mercury sits in a bulb on the bottom of the thermometer and rises up a thin capillary as the temperature rises. Suppose a mercury thermometer contains 3.450 g of mercury and has a capillary that is 0.200 mm in diameter. How far does the mercury rise in the capillary when the temperature changes from 0.0 °C  to 25.0 °C ? The density of mercury at these temperatures is 13.596  g/cm^3 and 13.534 g/cm^3 , respectively.
Express your answer with the appropriate units."
     This one I couldn't even figure out how to approach. I went back to it after the test was over and still couldn't figure it out.
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DrCMS

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Re: Tough questions
« Reply #3 on: October 12, 2012, 11:47:20 PM »

I get an answer between 2 and 3 ft for Q1
I'm not sure what you are doing after the first conversion to mm3 but you've not included the area of Texas in square miles at all.

I get an answer more between 7 and 8kg for Q2
Without seeing your working i can not see where you have gone wrong.

For Q3 you need to work out the height differences of two columns of Hg given the diameter and the total volume from the two different densities.
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