How many ml of a 17% w/v concentrate of benzalkonium chloride should be used in preparing 300 ml of a stock solution such that 20 ml diluted to 1L will yield a 1:5000 solution?

4 marks

Start from the end and work back.

1:5000 means 1 g in 5000 ml = 0.2 g in 1000ml

This means 20 ml of the stock solution contained 0.2 g of benzalkonium chloride.

0.2g/20ml = 0.3g/30ml = 3g/300ml

How many ml of 17% concentrate contains 3 g:

17%w/v means 17 g per 100ml

17g * (3/17) = 3g

100ml * (3/17) = 17.6ml