How do I measure caffeine powder?

I accidentally bought 85 grams of pure caffeine powder instead of pre-workout. I bought it from a chemistry lab that sells food supplements. It was so cheap compared to pre-workouts they sell on supplement websites. It didn't come with a measure or anything just white powder in a little tub. I want to take 400mg, how do I measure it? How many mg is a teaspoon of it? How much water do I need?