# Help with calculating the number of milligrams present in the solution

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The problem:

A liter of a mixture pf 0.4 Sodium Chloride and 0.2M Potassium Chloride.

I really don't know what to do with the compounds present in the solution.  Do I need to add them? I'm really lost. This is the last problem in the activity, the others are easy to understand and solve, but for me this one is hard to comprehend.

Maybe I need to calculate for their masses individually? What do you guys think?

Edited by popcornfrenzy
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58 minutes ago, popcornfrenzy said:

A liter of a mixture pf 0.4 Sodium Chloride and 0.2M Potassium Chloride.

This is not a question

Nor is it a complete statement.

So first you need to get is a full and accurate statement of the question and the values of the concentrations concerned.

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That's the full statement of the problem Sir Studiot and I really don't know what to do at this point. I kinda think it's more a given rather than a problem.

Edited by popcornfrenzy
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9 hours ago, popcornfrenzy said:

That's the full statement of the problem Sir Studiot and I really don't know what to do at this point. I kinda think it's more a given rather than a problem.

It looks to me like the sort of 'question' you find in texts on Pharmacy or Pharmaceutical calculations.

Very often there is a general question such as  "For each of the following mixtures of 100 mL of each, calculate the number of milligrams of each ion present in the solution mixture"

This is followed by a list of drill questions with different solutions.

By itself it is incomplete.

'pf' or percentage fraction is generally used by Pharmacists to mean grammes per litre or g/L and should be writen 0.4% which means it contains 4 grammes per litre of solution.

For example see here

I think the important issue with this question is that your strengths are in different units so you must convert to one or the other to obtain the concentrations in the resulting mixture.

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Okay noted Sir I'll try to solve it with the best of my ability thank you for the example

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My assumption is that it is asking you for the masses of each individually to make up a litre of solution with those concentrations and that the first number is meant to be in M. The last bit is an assumption though so I would check that. How much is also kind of vague and not stated but the only way I can think to answer it (it isn’t even really a question it’s just a statement) with that information is with masses.

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I'm confused by "pf". I didn't know "pf" (as a unit of concentration).

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3 hours ago, joigus said:

I'm confused by "pf". I didn't know "pf" (as a unit of concentration).

Common typo for “of” which is how I read it. Clarification needed.

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7 hours ago, swansont said:

Common typo for “of” which is how I read it. Clarification needed.

Thanks @swansont. It should have been obvious to me, but the lack of "M" after 0.4 plus the typo really confused me. The only way I can make sense of this is that both are molarities, as @hypervalent_iodine says, and you have to solve for the respective number of moles, but also for the volumes of both solutions, that must add up to 1 L. The way I see it, the problem is underdetermined, as @studiot suggests, as you would have 4 unknowns and only 3 equations:

$\frac{n_{1}}{V_{1}}=0.4$

$\frac{n_{2}}{V_{2}}=0.2$

$V_{1}+V_{2}=1$

Edited by joigus
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Aren’t they mixed together? So you have 0.2 moles of K, 0.4 moles of Na and 0.6 moles of Cl?

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If M is "moles", that may be it. Then the number of moles of Cl would have to add up to .6 and the $$V_1$$ and $$V_2$$ that I mentioned don't enter into it. "M" generally represents moles per liter though... I hate trying to guess what they're asking. 😆

I hesitate to say that must be it.

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The volume is 1 liter.

The issue is how you got there, because as all these posts point out, there are multiple valid interpretations. But assuming it’s a proper HW question, one can reasonably discard interpretations that are unsolvable.

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As I see it, @popcornfrenzy has not yet told us the entire question, as it is writen in the book or wherever.

How do we know that we have to calculate a number of milligrams ?

This was nowhere stated in the supposed complete copy of the question.

I have suggested it is of the sort in my attachment, starred to show how common this practice is.

So yet again I ask for the full question.

If you are not confident with these, there are many books of nothing but practice (drill) questions with answers.

Practice makes perfect.

And we do want our Baristas and Pharmacists to give us the right cocktail every time don't we ?

🙂

Edited by studiot
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On 6/10/2021 at 5:13 PM, popcornfrenzy said:

The problem:

A liter of a mixture pf 0.4 Sodium Chloride and 0.2M Potassium Chloride.

I really don't know what to do with the compounds present in the solution.  Do I need to add them? I'm really lost. This is the last problem in the activity, the others are easy to understand and solve, but for me this one is hard to comprehend.

Maybe I need to calculate for their masses individually? What do you guys think?

Okay, you have 0.2 Moles of Potassium and 0.4 WHAT of Sodium Choloride? 0.4 Moles?   You say "I really don't know what to do with the compounds present in the solution."  Well what are you trying to do?  What question are you trying to answer?  Your first sentence is a statement of fact about a situation.  So what?  There are no instructions to do anything and no question is asked. You say it is "the last problem in the activity.  What were the other problems?  Did they ask you to do something or answer a question?  Is it possible that part of this was cut off?

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