Basically there is not a conversion factor for between liters and grams. Liters is a volume measurement and grams is a mass/weight measurement. For any particular substance, you would need to know how many grams (or kilo-grams) there are in a liter of the substance. This would give you a conversion factor for that substance only (in the case you cited would be shampoo). So, if there is 50 grams per liter for shampoo, the conversion would be:

0.050kg/l x 5 liters = 0.25 kg

So, 5 liters of shampoo is equivalent to 0.25kg of shampoo.