[istrydummy]

1.
A solution of compound X of concentration 0.5 M is diluted 20 times. Calculate the number of mmoles of X present in 20 ml of the resulting solution.

My Method:
0.5M X 20 = 10 M ,therfore moles = 10 M x 0.02 L = 0.2 moles therefore = 2x10e-4 mmoles.
Is this correct?
 

2.
2 ml of a solution of X of concentration 0.30 mM was added to 6 ml of water. What is the number of mmoles of X in 0.50 ml of this solution.

My method: 0.05 ml x 0.30 = 0.15   therefore 0.15 x 6 = 0.9  therefore  = 9x10e-4 mmoles
Is this correct?

 

3.
20 grams of X is dissolved in a total of 500 ml. 100 ml of this solution is diluted 20 times. How many mmoles are present in this solution (Mr of X is 80)?

My method : Moles of X = 20/80 = 0.25   in 500 , and i dont know where to go from here?!


Thank you for your time guys.

[AWK]

1. This is dilution , not concentration
wrong recalculation moles to milimoles
2.0.5 ml = 0.05 ml
moles to milimoles - the same error as in 1
3. Which solution - before dilution or after dilution?

[istrydummy]

1. This is dilution , not concentration
wrong recalculation moles to milimoles
2.0.5 ml = 0.05 ml
moles to milimoles - the same error as in 1
3. Which solution - before dilution or after dilution?

1) I understand that there is a dilution, "0.5 M is diluted 20 times", but dont know what i am doing wrong.

2) Sorry. I ment to write 0.5 instead of 0.05. But does the answer seem fine if as 900mmoles?

3)I think its after dilution. But i dont know where i go from there or if i have the right assumptions. Could you show me how to solve it?

Thanks friend.

[Borek]

1) I understand that there is a dilution, "0.5 M is diluted 20 times", but dont know what i am doing wrong.

Diluted 20 times doesn't mean "concentration is higher 20 times" - exactly the opposite.

2) Sorry. I ment to write 0.5 instead of 0.05. But does the answer seem fine if as 900mmoles?

0.9 mmole, and it is wrong. Calculate new concentration first.

3)I think its after dilution. But i dont know where i go from there or if i have the right assumptions. Could you show me how to solve it?

0.25 mole in 500 mL - what concentration it is?

Question asks "How many mmoles are present in this solution" - if no volume is given, that means entire solution. Does adding water to the sample change number of moles of solute?

[istrydummy]

1) I understand that there is a dilution, "0.5 M is diluted 20 times", but dont know what i am doing wrong.

Diluted 20 times doesn't mean "concentration is higher 20 times" - exactly the opposite.

2) Sorry. I ment to write 0.5 instead of 0.05. But does the answer seem fine if as 900mmoles?

0.9 mmole, and it is wrong. Calculate new concentration first.

3)I think its after dilution. But i dont know where i go from there or if i have the right assumptions. Could you show me how to solve it?

0.25 mole in 500 mL - what concentration it is?

Question asks "How many mmoles are present in this solution" - if no volume is given, that means entire solution. Does adding water to the sample change number of moles of solute?

1) Is the answer 0.5 mmoles?

2) Could you explain about " calculating the new concentration"? i dont understand what you are referring to

3) The concentration of 0.25mole in 500 mL is 0.5 m/L.   
Is it right if i do 20(mass)/80(Mr) to get number of moles of X and multiply it by 0.5 to get 0.125mmoles?

[Borek]

1) Is the answer 0.5 mmoles?

Yes. Not idea if you have calculated it or guessed, but that's the correct answer.

2) Could you explain about " calculating the new concentration"? i dont understand what you are referring to

You have added 2mL to 6 mL - that's dilution. Calculate concentration of solution after mixing.

3) The concentration of 0.25mole in 500 mL is 0.5 m/L.

Correct.

Is it right if i do 20(mass)/80(Mr) to get number of moles of X and multiply it by 0.5 to get 0.125mmoles?

No, and no idea what you trying to do. IMHO you are just juggling numbers. First you have calculated you have 0.25 moles of substance, now you think it is 0.125 mmoles.

[istrydummy]

2) Is the new concentration 0.30 mM X 6 = 1.8 mM?
Then we do 1.8 X 0.5 to get 0.9mMoles?

3) I just dont understand why they give us the Mr of X, or say that 100ml of the solutuon was diluted 20 times.
When i calculated the concentration to be 0.5m/L do i multiply it by 5? Becuase 100/20(Dilution ) = 5
Thefore 0.5m/L X 5 = 2.5 m/L ? If yes, then what step to i do? If not what am i missing?

Thanks

[Borek]

2) Is the new concentration 0.30 mM X 6 = 1.8 mM?

No, and now it seems to me that you don't understand the underlying principle. Dilution calculations are based on mass preservation - whatever was present in the solution BEFORE dilution is present AFTER. Everything you do must take it into account.

http://www.chembuddy.com/?left=concentration&right=dilution-mixing