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Given percent composition, find mass per litre of solution

  • Thread starter brbrett
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  • #1
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Homework Statement


A 15% by mass solution has a density of 1.2 g/mL. What is the mass per liter concentration of this solution?
→ (a) 180 g/L (this is the answer, pretend you don't know)
(b) 150 g/L
(c) 125 g/L
(d) cannot be determined from this information

Homework Equations


Density
Possibly percent composition

The Attempt at a Solution


I don't know g/L.
D=m/v
I assume 15% of the solution is 15g (This is clearly not the case, or I wouldn't be here)
1.2=15/v
v=12.5mL
Convert mL to L
15g/0.0125mL
1200g/L <---Not even on the answer list, and sounds unreasonable anyway

Without straight telling me the answer, how does one approach a question such as this?
 

Answers and Replies

  • #2
Charles Link
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It's not the best problem in my opinion because the solution has a mass of 1200 g per liter, and if you subtract out the material that was dissolved, you have presumably what is water (when you are given a solution and the material is unspecified, water is usually implied), that must have a density that is at least 1020 grams per liter, and the volume might even be slightly less than a liter. (Sometimes the dissolved material will add volume. When it is removed, the volume could decrease.) 15% by mass of 1200 grams is 180 grams. I guess it is also fair to assume that the solution is not a water solution. Anyway, to help you with your calculations, there are 1000 ml (milliliters) in a liter. To do a percentage, you divide by 100. e.g. for 15%, its fractional value is 15/100=0.15.
 
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  • #3
SteamKing
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Homework Statement


A 15% by mass solution has a density of 1.2 g/mL. What is the mass per liter concentration of this solution?
→ (a) 180 g/L (this is the answer, pretend you don't know)
(b) 150 g/L
(c) 125 g/L
(d) cannot be determined from this information

Homework Equations


Density
Possibly percent composition

The Attempt at a Solution


I don't know g/L.
The problem statement clearly says the density of the solution is 1.2 g/mL. Don't you know that 1000 mL = 1 L?
D=m/v
I assume 15% of the solution is 15g (This is clearly not the case, or I wouldn't be here)
1.2=15/v
v=12.5mL
Convert mL to L
15g/0.0125mL
1200g/L <---Not even on the answer list, and sounds unreasonable anyway
This is the mass per liter of solution. It is not the mass concentration, however.
Without straight telling me the answer, how does one approach a question such as this?
X% mass concentration of a solution means that:

the mass concentration per liter = the mass of 1 liter of solution * X / 100
 
  • #4
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Now that I think about it, my work above was kinda silly :p
Plugging into the equation, I get 180g/L from 1200x(15/100), which is the correct answer (which is just 15% of that number).
What is mass concentration exactly? Is it simply the the part of the solution we are looking at, which implies the rest of the solution is water/other substances?
 
  • #5
SteamKing
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Now that I think about it, my work above was kinda silly :p
Plugging into the equation, I get 180g/L from 1200x(15/100), which is the correct answer (which is just 15% of that number).
What is mass concentration exactly? Is it simply the the part of the solution we are looking at, which implies the rest of the solution is water/other substances?
Mass concentration is the amount of a substance dissolved in some sort of solute to make a solution. The solute can be water or any other liquid.

Concentration can be defined in several different ways:

https://en.wikipedia.org/wiki/Concentration
 
  • #6
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The mass concentration they are looking for is the mass of solute per liter of solution.
 
  • #7
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Got it. Unfortunately I only get to use Molarity in my current year of chemistry, so it's good that I now know what mass concentration is. Thanks for the help!
 

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