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Topic: Calculating number of grams of solute from molal concentration  (Read 504 times)

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Offline DilutedBrain

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Calculating number of grams of solute from molal concentration
« on: February 13, 2020, 08:58:22 AM »
The Problem:Calculate the number of grams of solute necessary to prepare 700 grams of 0.6m H2SO4 solution.

The attempt: I tried to find the answer using the formula:

grams solute = molality * Molecular weight of H2SO4*L solvent

...until I realized there is no solvent in the problem. I tried to find the kilograms of solvent but there's no way I can find it without grams of solute or the weight percent. What formula should I use? Should I convert the molal concentration into molar concentration then find the grams of solute from there?

Offline AWK

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Re: Calculating number of grams of solute from molal concentration
« Reply #1 on: February 13, 2020, 09:14:47 AM »
Calculate the mass of sulfuric acid needed to make 0.6m from 1kg of water, calculate the mass of this solution, calculate the scaling factor to 700g and apply it to the previously calculated mass of sulfuric acid.
Show your result.
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Offline DilutedBrain

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Re: Calculating number of grams of solute from molal concentration
« Reply #2 on: February 13, 2020, 10:07:36 AM »
So... this is what I did

1. Calculate the mass of Sulfuric acid...

mass H2SO4 = O.6m * 98 g/mol (MW of H2SO4) * 1000 grams of water

I got 58,800 g H2SO4

2. Calculate the mass of this solution

I assumed the H2SO4 solution so I based the answer to the given of 700g H2SO4 solution. I don't get this one so I assumed this completely.

3. What I did for the scaling factor is this

58,800 grams/700 grams = 84 grams

...did I got this one right? or I failed?  ???

Offline AWK

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Re: Calculating number of grams of solute from molal concentration
« Reply #3 on: February 13, 2020, 10:13:54 AM »
The scaling (down) factor is 700/1058.8
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Offline DilutedBrain

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Re: Calculating number of grams of solute from molal concentration
« Reply #4 on: February 13, 2020, 10:17:46 AM »
Ohh... May I ask where the 1058.8 came from?

Offline AWK

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Re: Calculating number of grams of solute from molal concentration
« Reply #5 on: February 13, 2020, 10:19:58 AM »
1000+58.8
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Offline DilutedBrain

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Re: Calculating number of grams of solute from molal concentration
« Reply #6 on: February 13, 2020, 10:27:15 AM »
1000+58.8
ohh okay thank you.

So I got the answer of 0.66. Then I multiplied it to the mass of the H2SO4 and got 38.8 kg. Is this it or there's another step that I should take?  ???

Offline AWK

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Re: Calculating number of grams of solute from molal concentration
« Reply #7 on: February 13, 2020, 10:29:21 AM »
How you diminish 58.8 g to 38.8 kg
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Offline DilutedBrain

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Re: Calculating number of grams of solute from molal concentration
« Reply #8 on: February 13, 2020, 10:35:26 AM »
How you diminish 58.8 g to 38.8 kg

58.8 * 0.66 = 38.8

Offline AWK

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Re: Calculating number of grams of solute from molal concentration
« Reply #9 on: February 13, 2020, 10:36:02 AM »
The rounding is a bit incorrect.
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Offline DilutedBrain

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Re: Calculating number of grams of solute from molal concentration
« Reply #10 on: February 13, 2020, 10:41:18 AM »
The rounding is a bit incorrect.

Ohh... I got 38.808. I'll put it in 2 sig fig just to be sure.

So... I got 39.

Offline AWK

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Re: Calculating number of grams of solute from molal concentration
« Reply #11 on: February 13, 2020, 10:44:10 AM »
0.6*H2SO4*700/(1000+0.6*H2SO4) = 38.9029
so 40, 39 or 38.9 depending on the number of significant digits (1, 2 or 3)
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Offline DilutedBrain

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Re: Calculating number of grams of solute from molal concentration
« Reply #12 on: February 13, 2020, 10:46:11 AM »
Ohh okay I get it now. Thank you so much for the help  :)

Offline Borek

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Offline MNIO

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Re: Calculating number of grams of solute from molal concentration
« Reply #14 on: February 14, 2020, 03:10:21 PM »
you know this
  molality = 0.6m H2SO4 = moles solute / kg solvent
  mass solution = 700g
  mass solvent = mass solution - mass solute

you could always put in the effort and derive an equation to relate m, mass solution
mass solvent, etc.... but... the easy way is just to assume a known value of something
then calculate away.  Since this is molality, let's assume we have 1 kg of solvent

in which case, we would have
  1kg solvent
  1kg solvent * 0.60 mol H2SO4 / kg solvent = 0.60 mol H2SO4
  mass H2SO4 mol = 0.60 * 98.09 g/mol = 58.85g H2SO4
  total mass solution = 1kg + 0.05885kg = 1058.85g

and now we know 58.85g H2SO5 = 1058.85g solution... the rest is trivial via dimensional analysis

   700g solution      58.85g H2SO4
 ------------------ x ------------------- = 38.90g H2SO4
            1              1058.5g solution

*********
remember that trick of assuming 1kg of solvent or 1L of solution, etc.  It works very well for molarity / molality type problems.

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