#9 on Midterm Practice Review (unicorn)

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paulacamara1E
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#9 on Midterm Practice Review (unicorn)

Postby paulacamara1E » Sun May 06, 2018 9:42 pm

On the practice review that one of the UA's prepared, the last question (number 9) is about molarity and volume. Since M is mol/L I assumed that we had to convert the given volume from mL to L, however, that is not the case. All of the calculations are made with mL, why is that so? And how do I know when I should or should not convert?

Chem_Mod
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Re: #9 on Midterm Practice Review (unicorn)

Postby Chem_Mod » Sun May 06, 2018 10:38 pm

The typical unit for concentration is molarity (M = moles/Liter), so L would be the typical unit that most would employ. However, as long as the units are consistent to the numbers being calculated they mean the same thing.

404975170
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Re: #9 on Midterm Practice Review (unicorn)

Postby 404975170 » Sun May 06, 2018 10:42 pm

It is usually always best to automatically convert to L in most problems when you see mL since this is the unit the answer needs to be in for most problems. This question does not specify the units the answer has to be in so since the question gives the volume in mL it is just easier to find the answer in mL to find the answer for example:
If you had to find the volume for 10 M KmNO4 that's needed to prepare 5000 mL of 15 M KmNO4 then the answer would be 7500 mL
(10M)(_____mL)=(15M)(5000mL)
but say you had converted this to L when you first saw the problem you would get
(10M)(____L)=(5L)(15M) then you would still get 7.5 liters which is still the same as 75000 mL since it's still the same ratio for both.
If you find it easier then just always convert to liters to avoid making a mistake.

Yadira Flores 1G
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Re: #9 on Midterm Practice Review (unicorn)

Postby Yadira Flores 1G » Sun May 06, 2018 10:42 pm

Also I always convert the given mL to L and do the calculations, unless it specifically states the answer is wanted in mL. Also, he posted the answers under the topic and the answer he gave had the final units in L.

Isabel Jabara 1C
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Joined: Fri Apr 06, 2018 11:03 am

Re: #9 on Midterm Practice Review (unicorn)

Postby Isabel Jabara 1C » Sun May 06, 2018 10:43 pm

It doesn't matter if you convert mL to L before you solve the problem or after you solve it, as long as you use the same unit the whole time you are solving the problem. Answers are usually given in L, but the UA gave the answer in mL.


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