[JackT]

Question: How many grams SiO2 are needed in 1 liter of water to yield a concentration of 400 mg of SiO2 per 1 ml of water?

[Arkcon]

I'd like to welcome you, JackT:, the the Chemical Forums, but I'd like to ask you to read the Forum Rules{click}.  We like to help you learn to help yourself.  Your question ask for grams per liter, to give a known milligrams per mililiter.  Can you begin to make some of those conversions for us?  You're going to face more difficult questions sooner rather than later, and you need the practice,

[JackT]

C=m/v
So, I have a ratio of 400:1
Making the correct concentration 400 g SiO2 per 1 L of water?

[PhotoElectroMaterials]

your answer is correct.  And your  problem is more  about maths  and unit conversion than chemistry.  You can simply multiply 400mg by 1000  since you are scaling  up from 1ml  to 1000ml.

[Enthalpy]

Is SiO₂ any soluble in water? And if not, would we define a concentration?