How many milligrams are contained in 25mL?

Answer 1

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 25 mL of air. How many milligrams? Next consider 25 mL of water. How many milligrams?

The masses of same volumes of air and water 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 millilitres 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 kg/metre3