Liquids are measured in litres, pints, fluid ounces etc not grams. The gram is the unit of measurement for 'non'-liquids. 1/10 of a 'kilo'gram would be 100 grams (1/10 of 1000). Kilo means in 'thousands' ie kilowatt, kilometre, kilobyte etc.

100 grams or 10% of a kilogram

Leon was right that it is very unusual to measure liquids by weight, it is one of the beauties of the metric system that one kg of water is 1 litre, making it much simpler. Maybe you would have been better to say 1kg of rice, or 1kg of flour? As both said, 100 grams would be the best way. One tenth of a kilo(gram) would be next best. It would be very strange to say 10 percent of a kilo. I've never ever heard anyone say that. Which you would use would depend on the context and the quantity. If it was something exact, like a scientific paper, or a recipe, it would always be xxx grams. If it were in a shop, you would probably say 'a quarter of a kilo' rather than 'two hundred and fifty grams', just because it is shorter.