What is the original % after dilution?

Question

Ig is diluited in 99g of water. 0.4g is then taken from this diluted mixture and mixed with 100g of paste. what is the original % in the paste?

Please help, I have tried posting this before but it seems o=to get deleted.

thnx

Answer 1

There are a number of reasons why this question can't be answered as posed. Firstly you don't say what the substance is - some things ( e.g. NaOH) will reduce the volume of water when added to it, so that is why solutions are made up to a volume - not so many grams of solute + so many grams of solvent. Also you don't say whether by % you mean w/v or v/v or w/w

Answer 2

1 g solute per 100 g of solution gives a 1 % (w/w) solution.

0.4 g of the 1 % solution is then diluted to a final mass of 100.4 g. The diluted w/w % is:

1 % x (0.4 / 100.4) = 0.00398 % (w/w)