10ML is how many milligrams?

Answer 1

There can be no equivalence.

A milligram is a measure of mass. A millilitre is a measure of volume. The two measure different things and, according to basic principles of dimensional analysis, conversion from one to the other is not valid.

If you are not convinced, consider 100 millilitres of air. How many milligrams? Next consider 100 millilitres of Mercury. How many milligrams?

The masses of equal volumes of the two substances will clearly be very different. So there is no direct conversion between mass and volume: you need to know the density of the substance to enable you to carry out the conversion.