I am hoping someone can mark my calculatlions and tell me if I am correct or not?

if I need to make a 400ml, 5% w/w solution of 98% sulphuric acid/water, how much sulphuric acid will I need to add?

The weight of 98% sulphuric acid equals (approx) 1.84grams per 1 ml of liquid. The weight of water equals 1gram per 1ml of liquid.

Therefor a weight/weight solution of 5% in 400ml (20grams) would only require 10.86ml of 98% sulphuric acid.

20g divided by 1.84g = 10.86ml of liquid

Am am I correct in my calculations?

Any help is appreciated

You need to know the DENSITY of 5% aqueous (w/w) sulfuric acid. You have to look it up:

http://myh2so4.norfalco.com/documents/NorFalco_H2SO4Property02.pdf

The answer happens to be 1.0334 g/mL, so 400.0 g of this solution actually weighs 413.4 g and contains 20.67 g of pure sulfuric acid. So you need 21.09 g (11.46 ml) of 98% sulfuric acid and 392.3 g water.