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Concentrations and volumes

A volume of 7cm3 of 20mM sodium chloride solution was diluted 1 in 30 with water.

1. What volume of water was added (in cm3)?
2. What is the concentration of the diluted sodium chloride (in mM)?
3. How many grams of glucose would you need to prepare 50cm3 of a 36% solution?
what do you think? How do you do dilutions? Or calculate concentrations?
Original post by gingerbreadman85
what do you think? How do you do dilutions? Or calculate concentrations?


The only thing I've got from the first question is that volume = concentration/moles.

I'm presuming moles = 20/7 = 2.8571..mM/cm3

then my mind has gone blank
well what volume of the initial solution would be used to make up 30ml of the diluted solution?

then how would you convert that into the molarity of the new solution?

%s are normally given weight by volume, so a 50% solution is 50g of solute in 50g of solvent.
(edited 13 years ago)
Original post by gingerbreadman85
well what volume of the initial solution would be used to make up 30ml of the diluted solution?

then how would you convert that into the molarity of the new solution?

%s are normally given weight by volume, so a 50% solution is 50g of solute in 50g of solvent.


I got 10.5cm3 for the first part, is that right?
not quite. If you use all of the initial solution, how much water will you have to add to make it 1/30?
Original post by gingerbreadman85
not quite. If you use all of the initial solution, how much water will you have to add to make it 1/30?


6.7cm3???
if you are making 30cm3, then 1/30 would be 1cm3 of your original solution

how much water do you have to add to your original solution then?
Original post by gingerbreadman85
if you are making 30cm3, then 1/30 would be 1cm3 of your original solution

how much water do you have to add to your original solution then?


The original solution is 20mM right? so 1/30 of 20 is 0.6667

20 - 0.6667 = 19.333..
Anyone....please!! :frown: :frown:
Original post by I Have No Imagination
A volume of 7cm3 of 20mM sodium chloride solution was diluted 1 in 30 with water.

1. What volume of water was added (in cm3)?
2. What is the concentration of the diluted sodium chloride (in mM)?
3. How many grams of glucose would you need to prepare 50cm3 of a 36% solution?


1. If it was diluted 1 to 30 with water the final volume is 30 x 7 = 210 ml

2. If you start with 7 you have to add 210 - 7 = 203 ml of water

The diluted solution will now be 1/30 times the original strength = 20/30 = 0.67 mM

3. A 36% solution contains 36g in 100cm3

you only need 50 cm3 so how many grams are needed?

Hint: 50 is one half of one hundred...
Original post by charco
1. If it was diluted 1 to 30 with water the final volume is 30 x 7 = 210 ml

2. If you start with 7 you have to add 210 - 7 = 203 ml of water

The diluted solution will now be 1/30 times the original strength = 20/30 = 0.67 mM

3. A 36% solution contains 36g in 100cm3

you only need 50 cm3 so how many grams are needed?

Hint: 50 is one half of one hundred...


ahhh now it makes sense!! thank you sooooo much!!!! :smile:

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