How do you convert 147 mL to grams?

You don't.

A kilo 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 14.7 ml air. How many grams? Next consider 14.7 ml of water. Will its mass be the same? 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 water but that is only approximately true. Until 1964 a litre was defined as the volume of one kilogram of pure water at 4 deg Celsius and at a pressure of 760 millilitres of Mercury. With that definition 14.7 millilitres of water (under those conditions) would have had a mass of 14.7 grams. But that definition of a litre was abandoned in favour of 1 litre =1000 cubic centimetres.