How many mg is 0.45ml?

Answer 1

None.

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 a millilitre of air. How many milligram Next consider a millilitre of lead. How many milligram?

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.

Some people still believe that there is a conversion in relation to pure water but that is only approximately true. Until 1964 (nearly 50 year ago!) a litre was defined as the volume of one kilogram of pure water at 4 degrees Celsius and at a pressure of 760 millimetres of Mercury. With that definition a conversion would have been valid - but only for pure water and only under those conditions. In any case that definition of a litre was abandoned in favour of 1 litre =1000 cubic centimetres.

In fact the density of pure water, at 4 deg C and 760 ml of mercury is 999.9720 mg/litre. At other temperatures it is less dense: at 20 deg C it is 998.2071 mg/litre.