# IMAT 2021 question 50

100mL of a solution contained 3,2g of potassium nitrate.
25mL of this solution was added to an empty 250mL volumetric flask. Distilled water was added up to the 250mL mark and the flask was shaken to ensure that mixing was complete.
A pipette was used to transfer 25mL of the resulting solution from the volumetric flask to an empty conical flask.

What is the concentration of the potassium nitrate solution in gL⁻¹ in the conical flask?

Hi!
The answer is 3,20gL⁻¹, what’s the calculation here?

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Hi! I did the calculation hope this helps! Just a tip, the last line is meant to confuse you because in the calculation it’s a useless value and the concentration doesn’t change when you extract 25ml of the same solution.

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Hi Raya!
Can you please tell me why the concentration doesn’t change when we extract some amount from the solution?
Thanks beforehand:)

Okay so since the salt is dissolved in the solution, the salt is distributed EQUALLY throughout the whole solution.
So when we extract let’s say 50 ml out of 100 ml solution, the amount of solute in that solution will ALSO halve, there will be half the amount of salt in that 50 ml. So when you take the equation concentration = mass / volume, if both mass and the volume decrease by half, the concentration will remain the same.

eg:
20 g of NaCl in 100 ml of water gives a concentration of 200 g/L

Now we take 50 ml of that solution, the mass will also be half of that - 10 g in 50 ml - also 200g/L

I hope this helps! Let me know!

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thank you so much for clearing it out this helps a lot:)

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Hi!

I thought we always used C=n/V
When is C=m/V used? For dilutions?

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Hi!
These are actually the different ways of expressing the concentration of a solutions and we use c= n/v frequently unless it’s mentioned in the question like this one to use the other formula:)

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