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# HELP with concentration

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Why bother with a post grad course - waste of time? 17-10-2016
1. I found a question in my revision guide and I really didn't understand how to get the answer. I've tried working backwards and looking online, but it's no good -.-
Can someone help?! If you can, write the working out please!

The question is:
explain 5cm^3 of a soltion has a concentration of 1 mol/dm^3. How much water should be added to the solution in order to make it have a concentration of 0.1mol/dm^3?

2. 1. Convert the 5cm³ into dm³.
There are 1000cm³ in 1dm³
5cm³ ÷ 1000cm³/dm³ = 0.005dm³.

2. You find the amount of the substance for which the concentration will change
intiial concentration (mol/dm³) = amount (mol) ÷ initial volume (dm³)
amount = concentration ×volume
amount = 1mol/dm³ × 0.005dm³
amount = 0.005mol

3. No more of the substance will be added, but more water will be added, so the amount will stay the same.
final concentration (mol/dm³) = amount (mol) ÷ final volume (dm³)
volume = amount ÷ concentration
volume = 0.005mol ÷ 0.1mol/dm³
volume = 0.05dm³

4. Remember you aren't working out 'final volume', you are working out 'change in volume'
change in volume = final volume - initial volume
change in volume = 0.05dm³ - 0.005dm³
change in volume = 0.045dm³

5. You can convert the 0.045dm³ to cm³ (I don't know if the question needs you to do this)
There are 1000cm³ in 1dm³
0.045dm³ × 1000cm³/dm³ = 45dm³

You would need to add 45cm³ of water
3. (Original post by emperorCode)
1. Convert the 5cm³ into dm³.
There are 1000cm³ in 1dm³
5cm³ ÷ 1000cm³/dm³ = 0.005dm³.

2. You find the amount of the substance for which the concentration will change
intiial concentration (mol/dm³) = amount (mol) ÷ initial volume (dm³)
amount = concentration ×volume
amount = 1mol/dm³ × 0.005dm³
amount = 0.005mol

3. No more of the substance will be added, but more water will be added, so the amount will stay the same.
final concentration (mol/dm³) = amount (mol) ÷ final volume (dm³)
volume = amount ÷ concentration
volume = 0.005mol ÷ 0.1mol/dm³
volume = 0.05dm³

4. Remember you aren't working out 'final volume', you are working out 'change in volume'
change in volume = final volume - initial volume
change in volume = 0.05dm³ - 0.005dm³
change in volume = 0.045dm³

5. You can convert the 0.045dm³ to cm³ (I don't know if the question needs you to do this)
There are 1000cm³ in 1dm³
0.045dm³ × 1000cm³/dm³ = 45dm³

You would need to add 45cm³ of water
Thanks for the help! It's so much more clear now!
4. (Original post by DangIt)
Thanks for the help! It's so much more clear now!
No problem... helped me revise concentrations for myself - it's a win for both of us.
(Thanks for the rep, means a lot to me and for the practise)
5. (Original post by emperorCode)
No problem... helped me revise concentrations for myself - it's a win for both of us.
(Thanks for the rep, means a lot to me and for the practise)
XD totally fine

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