Attachment: Download

Issue Description:

The UnitConcentrationMass offers an initializer for the unit millimolesPerLiter that takes a gramsPerMole parameter. The official documentation states that the coefficient to the base unit (g/L) is 18 * gramsPerMole.

The factor of 18 seems superfluous, and yields an erroneous result when converting a Measurement in millimolesPerLiter to a value in gramsPerLiter. Also, the conversion is off by 3 orders of magnitude (10e3)

A simple dimensional analysis can show that the factor 18 is superfluous and the conversion millimole to mole (* 10e3) is not accounted for.

Example:

• NaCl (table salt) has a molecular mass of 58 g/mol (see Wikipedia)

• Let's imagine a solution with a concentration of 1 mol/L (i.e. 1000 millimoles per Liter)

• With 1 mol/L and 58 g/mol, the same concentration of 1 mol/L is equivalent to 58 g/L

As a result, to conversion coefficient for millimolesPerLiter to the base unit (g/L) should be "gramsPerMole / 1000"

This 18 factor seems to be closely related to Blood sugar conversions (which is the only molecular mass unit added with HealthKit). See here how 1 mmol / l of blood sugar is equal 18 mg / dl.

See attached playground.