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A level biology question

I’m confused on this aqa a level biology question.
Describe how you would use a 1.0 mol dm solution of sucrose to produce 30 cm'3 of a 0.15 mol cm-3 solution of sucrose.

Answer: would add 4.5cm' sucrose solution to 25.5cm" distilled water.
I understand about finding the dilution factor but what’s tripping me up is the distilled water part. Can someone explain this question simply please. Thank you to anyone reading this.
Reply 1
Distilled water contains no solutes at all, so it is guaranteed to have no sucrose dissolved in it. You need it to keep the moles of sucrose the same (only from the 4.5cm^3 of sucrose solution you add) while increasing the volume of solution to the required 30cm^3
I had a rough time with working this out in my head, but I have a method for working it out now:

1.

First you need to find the scale factor of sucrose solution to see how many more times the new solution is diluted using conc. of the solution you want ÷ conc. of the solution you have: (0.15 ÷ 1 = 0.15 times more dilute).

2.

Then times your dilution factor by the total volume of the new solution to find the volume of solution you need: (0.15 x 30cm3 = 4.5cm3 of 1 mol dm3 sucrose solution).

3.

Finally, you just need to make up the solution to 30cm3 by topping it up with distilled water (or whatever solvent is used in the question). Work out this required volume by taking the volume of the first solution from 30cm3: (30cm3 - 4.5cm3 = 25.5cm3 of distilled water)

Thus to make 30cm3 of a 0.15M sucrose solution from a 1M sucrose solution, you need 4.5cm3 of 1M sucrose solution and 25.5cm3 of distilled water.

Hope this helps :smile:
Reply 3
Original post by didntask1
I had a rough time with working this out in my head, but I have a method for working it out now:

1.

First you need to find the scale factor of sucrose solution to see how many more times the new solution is diluted using conc. of the solution you want ÷ conc. of the solution you have: (0.15 ÷ 1 = 0.15 times more dilute).

2.

Then times your dilution factor by the total volume of the new solution to find the volume of solution you need: (0.15 x 30cm3 = 4.5cm3 of 1 mol dm3 sucrose solution).

3.

Finally, you just need to make up the solution to 30cm3 by topping it up with distilled water (or whatever solvent is used in the question). Work out this required volume by taking the volume of the first solution from 30cm3: (30cm3 - 4.5cm3 = 25.5cm3 of distilled water)

Thus to make 30cm3 of a 0.15M sucrose solution from a 1M sucrose solution, you need 4.5cm3 of 1M sucrose solution and 25.5cm3 of distilled water.
Hope this helps :smile:

Thank you so much this was helpful I know understand the rest but why was water needed the question didn’t ask for it.
Reply 4
Original post by zmp903
Distilled water contains no solutes at all, so it is guaranteed to have no sucrose dissolved in it. You need it to keep the moles of sucrose the same (only from the 4.5cm^3 of sucrose solution you add) while increasing the volume of solution to the required 30cm^3

Thank you so much this was helpful.

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