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### Topic: Question on Concentration  (Read 7082 times)

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#### derek0o0

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##### Question on Concentration
« on: May 03, 2009, 10:59:54 AM »
Q1 Given solution A concentration 1 in 50, solution B concentration 1 in 250.
What volume of each is required to produce 200ml of a 0.759% solution?

I have no idea how to do it except converting 1 in 5 to 2% and 1 in 250 to 0.4%

Q2 What is % saturation of 3M KCl?

Is this something with the RMM?

Help

#### Borek

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##### Re: Question on Concentration
« Reply #1 on: May 03, 2009, 11:30:25 AM »
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#### derek0o0

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##### Re: Question on Concentration
« Reply #2 on: May 03, 2009, 11:37:08 AM »
http://www.chembuddy.com/?left=concentration&right=dilution-mixing
i understand the concept but i duno how to apply into the question
thx

#### Borek

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##### Re: Question on Concentration
« Reply #3 on: May 03, 2009, 12:05:01 PM »
Assume vA and vB. Final volume is vA + vB, can you write mass balance for solute? That will give you two equations in two unknowns. Solve.

Second - especially taken out of context - doesn't make much sense to me. No idea what is a solubility of KCl (this have to be checked in tables). But I am not sure if my guess that the question is related to the solubility is right.
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#### derek0o0

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##### Re: Question on Concentration
« Reply #4 on: May 03, 2009, 12:24:44 PM »
Assume vA and vB. Final volume is vA + vB, can you write mass balance for solute? That will give you two equations in two unknowns. Solve.

Second - especially taken out of context - doesn't make much sense to me. No idea what is a solubility of KCl (this have to be checked in tables). But I am not sure if my guess that the question is related to the solubility is right.
i duno how to make up the 2 equations for Q1 lol
and for Q2...that is a question in the exam
it just gives you the RMM of KCl 74.6 and nothing else
so i think the calculation is nothing do with the solubility

#### ARGOS++

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##### Re: Question on Concentration
« Reply #5 on: May 03, 2009, 12:29:41 PM »

Dear derek0o0;

The solution results in two equations with two unknown:
Here the first equation:   x ml   +   y ml   =  200.0 ml.

Are you able to spend the second equation in x and y and with the concentrations you have already calculated?

For Q2:  Do you know the definition of molarity; and what 3M means?

Good Luck!
ARGOS++

#### derek0o0

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##### Re: Question on Concentration
« Reply #6 on: May 04, 2009, 08:51:42 AM »

Dear derek0o0;

The solution results in two equations with two unknown:
Here the first equation:   x ml   +   y ml   =  200.0 ml.

Are you able to spend the second equation in x and y and with the concentrations you have already calculated?

For Q2:  Do you know the definition of molarity; and what 3M means?

Good Luck!
ARGOS++

I have solved Q1 thx
For Q2....3M means 3 moles of a substance dissolved in one litre
And I have another question to ask lol

Given 2 batches of syrup (batch X 1.32g/ml and batch Y 1.2g/ml).
What volumes of each are required to produce 400ml of a syrup of 1.035g/ml?

this looks similar to Q1..but i can't solve it using the same method

#### sjb

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##### Re: Question on Concentration
« Reply #7 on: May 04, 2009, 09:11:44 AM »
Are we assuming that for Q1 you were blending the two solutions together? Is that right, or were you supposed to evaluate the two solutions as separate, and so give two recipes?

Can you work out the answer if the two solutions were separate and just diluted with solvent?

#### ARGOS++

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##### Re: Question on Concentration
« Reply #8 on: May 04, 2009, 10:31:45 AM »

Dear derek0o0;

Your new problem has no solution, because the target density is too low (is the lowest).
Could it be that you did a tipping mistake for the densities?

But you are right, that with correct values for densities, the solution can be found/calculated exactly the same way as for Q1.

For Q2:  Yes! - and 3 mole KCl in 1000ml makes how much %?

Good Luck!
ARGOS++

#### derek0o0

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##### Re: Question on Concentration
« Reply #9 on: May 04, 2009, 12:04:08 PM »

Dear derek0o0;

Your new problem has no solution, because the target density is too low (is the lowest).
Could it be that you did a tipping mistake for the densities?

But you are right, that with correct values for densities, the solution can be found/calculated exactly the same way as for Q1.

For Q2:  Yes! - and 3 mole KCl in 1000ml makes how much %?

Good Luck!
ARGOS++

Do you mean the values for the densities are incorrect?
I am pretty sure I typed the correct values
For 2...I am totally confused

#### ARGOS++

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##### Re: Question on Concentration
« Reply #10 on: May 04, 2009, 12:30:58 PM »

Dear derek0o0;

You’re right. – Something with the densities must be wrong.

Think about: You can only create mixtures with a density between 1.32g/ml and 1.2 g/ml, because your starting solutions have these values. So it’s impossible to create a mixture of lower density than 1.2 g/ml.

For Q2:  How much grams are 3 mole of KCl? – Then you can calculate the %tage.

Good Luck!
ARGOS++

#### Borek

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##### Re: Question on Concentration
« Reply #11 on: May 04, 2009, 01:56:42 PM »
You can only create mixtures with a density between 1.32g/ml and 1.2 g/ml, because your starting solutions have these values. So it’s impossible to create a mixture of lower density than 1.2 g/ml.

That's assuming density changes are monotonic. They usually are, but not always.

Quote
How much grams are 3 mole of KCl? – Then you can calculate the %tage.

Which is not what the question asks for (especially assuming that answer given - 2.24% - is correct).
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#### ARGOS++

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##### Re: Question on Concentration
« Reply #12 on: May 04, 2009, 03:03:58 PM »
Dear Mr. Borek;

You can only create mixtures with a density between 1.32g/ml and 1.2 g/ml, because your starting solutions have these values. So it’s impossible to create a mixture of lower density than 1.2 g/ml.

That's assuming density changes are monotonic. They usually are, but not always.
As the question implies that both solutions contain the same solute and the identical solvent, so it is not only an assumption, so it will hold quite well. And a density of 1.035 g/ml is impossible with these both solutions.

For Q2:  I still believe there is a typo; it may be .3 M or 22.4%.  And I hoped that derek0o0 would realise this after calculation.

Good Luck!
ARGOS++

#### derek0o0

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##### Re: Question on Concentration
« Reply #13 on: May 04, 2009, 03:16:49 PM »
You can only create mixtures with a density between 1.32g/ml and 1.2 g/ml, because your starting solutions have these values. So it’s impossible to create a mixture of lower density than 1.2 g/ml.

That's assuming density changes are monotonic. They usually are, but not always.

Quote
How much grams are 3 mole of KCl? – Then you can calculate the %tage.

Which is not what the question asks for (especially assuming that answer given - 2.24% - is correct).
so do you have any idea of how to do this density question?
for Q2, 3mole = 74.6 x 3 = 223.8g in 1000ml
but how do you convert this into %?
thx