Chemistry question dilution

If a solution is diluted by 10 fold, surely it will be less concentrated? So shouldn’t the concentration of a solution be divided by 10 rather than multiplied? Thanks!

Well, yes.

Suppose we have ‘n’ moles of solute initially dissolved in ‘V’ dm^3 of solvent.

In which case, the concentration of solute would be n/V mol dm^-3

Suppose we were to dilute the solution by adding ‘9V’ dm^3 of solvent and nothing else, bringing the total volume to ‘10V’ dm^3. Our new concentration would be:

n/(10V) = 1/10 x (n/V) mol dm^-3
Or one tenth of the original concentration

The mathematics is irrefutable.

In one of the questions I did, the concentration was 10 mol/dm3 and at the beginning of the question, it said the solution was diluted by 20 fold with distilled water. Instead of dividing the concentration by 20 they multiplied by 20 instead but I don’t understand how? Thank you!

In one of the questions I did, the concentration was 10 mol/dm3 and at the beginning of the question, it said the solution was diluted by 20 fold with distilled water. Instead of dividing the concentration by 20 they multiplied by 20 instead but I don’t understand how? Thank you!

It might help if you could post a picture of this question.

If a solution is diluted by a factor of 20, then the concentration of the solution is reduced by a factor of 20. To calculate the new concentration, you would divide the original concentration by 20.
However, if the question asked for the amount of solute needed to make the diluted solution, then you would need to multiply the original concentration by the volume of the diluted solution. For example, if you have a 10 mol/dm3 solution and you want to make 1 dm3 of a diluted solution, you would need to multiply 10 mol/dm3 by 1 dm3 to get 10 moles of solute.

Sorry please find attached

I looks like you may have misread the question. It appears to be stating that the sample of pool water was diluted 20 fold, and then gives the value for the concentration of the diluted solution, and asks that you calculate the original concentration of the undiluted pool water.
(the question definitely could be a bit more precisely worded).

Ohh that makes more sense, thanks!

I looks like you may have misread the question. It appears to be stating that the sample of pool water was diluted 20 fold, and then gives the value for the concentration of the diluted solution, and asks that you calculate the original concentration of the undiluted pool water.
(the question definitely could be a bit more precisely worded).