If you are asked to dilute 67 mL of a 21% (w/v) sugar solution 1:7 how much solvent must be added? What is the sugar concentration in % of the new solution?

Answer 1

First thing you should do is write out the variables you are given, & some of them are not given directly.

Initial concentration: 21% (w/v)

Initial volume: 67 mL

Final concentration: ?

The final concentration is told in that it will be diluted 1:7 from the initial concentration. This means that the final concentration is 21/7 = 3%.

3% is the sugar concentration in the new solution, so that is one of your answers.

The other question asks for the volume of solvent needed to be added. I would use the formula:

C1*V1 = C2*V2

Where C is the concentration, V is the volume, 1 is the initial, & 2 is the final.

(21% * 67 mL)/ 3% = 469 mL

So, by solving this problem the way I did, you would take the 67 mL solution & add water to it, until the final volume reached 469 mL.

The question asks for the volume of SOLVENT added, so 469 is not your final answer. Remember that the solvent is what the solute dissolves in, usually water. The final volume was 469 mL, & you started with 67 mL. This means you added 469 - 67 = 402 mL of solvent.

I hope this helps.

Best of luck to you,

Michael G.