How many ml in 340 grams?

Answer 1

None.

A gram 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 grams? Next consider a millilitre of Mercury. How many grams?

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 (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 possible - 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.

So now the maximum density of pure water, at 4 deg C and 760 mm of mercury is 0.999 972 g/cc