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### Topic: Help on homework on molarity  (Read 1711 times)

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#### rumwrld

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##### Help on homework on molarity
« on: August 12, 2021, 04:57:03 AM »
A 0.50M solution of HCl is prepared mixing one 1M solution of HCl and another 0.1 M solution of HCl. What is V(1M solution) / V(0.1 solution) in which V is the volume of the solution.
Problem was originally in italian so I don’t know if I translated correctly.. I just can’t wrap my head around it.

Bonus exercise (I did this one and i got a different result from the book but I think mine is correct):

10 L of a solution 0.20M of NaCl is prepared mixing a 0.10 M and a 0.5 M solution of the same salt. Calculate the originary volumes of the two solutions.

#### Orcio_87

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##### Re: Help on homework on molarity
« Reply #1 on: August 12, 2021, 05:15:17 AM »
@rumwrld Did you heard about the cross concentration rule ? It gives information about in what volumes solutions must be mixed. Apply it here.

#### rumwrld

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##### Re: Help on homework on molarity
« Reply #2 on: August 12, 2021, 05:23:18 AM »
@rumwrld Did you heard about the cross concentration rule ? It gives information about in what volumes solutions must be mixed. Apply it here.
Never.. problem is my teacher gave us this homework and it’s from university and I go to high school lol.. she never explained these things to us

#### Borek

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##### Re: Help on homework on molarity
« Reply #3 on: August 12, 2021, 07:33:23 AM »
While there are some ready rules that can be used to speed up calculations, all such questions can be solved by following mass balance - whatever you put in the solution stays there - and assuming the final volume is the sum of volumes. That produces two equations in two unknows, write them, solve, problem done.

Technically the answer won't be correct, as volumes are not additive (masses are) - the final volume is almost never exactly sum of volumes. But as long as the solutions are reasonably diluted the error is negligible.
« Last Edit: August 12, 2021, 10:16:59 AM by Borek »
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