# Mole <-> Mass Conversion

In UCUM the mol unit is defined as dimensionless, more precisely 1 mol = 6.02214076*10^23. Here is their reasoning:

The rationale for removing the mole from the base is that the mole is essentially a count of particles expressed in a unit of very high magnitude (Avogadro's number). There is no fundamental difference between the count of particles and the count other things.

ucumate supports both conversions so depending on the provided context:

• 1 mol = 6.02214076*10^23
• 1 mol = X * g where X is a provided substance's molar mass coefficient which changes depending on the solution.

There is a property ucumate.enableMolMassConversion which can be used to allow or disallow the conversion application-wide. It is allowed by default.

Assuming the ucumate.enableMolMassConversion property is enabled, you can convert mol (or any unit that canonicalizes to mol) to a mass unit.

ucumService.convert("1", "mol", "g", "18.01528"); // 18.01528 is the substance coefficient in water

If you don't provide the substance coefficient, then the conversion will fail because ucumate will try to convert to the dimensionless unit.

ucumService.convert("1", "mol", "g"); // yields a BaseDimensionMismatch because mol->g without substance coefficient is not possible
ucumService.convert("1", "mol", "1"); // yields 6.02214076*10 {#23}

Conversely, if ucumate.enableMolMassConversion is set to false, then converting mole to gram will always fail

ucumService.convert("1", "mol", "g", "18.01528"); // yields a BaseDimensionMismatch because mol<->mass conversion is disabled application-wide

If you don't want to provide decimal number for the substance's molar mass coefficient you can add the ucumate-mol-support dependency.

<dependency>
    <groupId>io.github.fhnaumann</groupId>
    <artifactId>ucumate-mol-support</artifactId>
    <version>1.0.3-SNAPSHOT</version>
</dependency>

This lets you define the coefficient optionally by a different name, casNr, formular, etc.

// Use a casNr instead of a decimal number.
Converter.ConversionResult result = ucumService.convert("1", "mol", "g", "1309-37-1");
System.out.println(result); // conversionFactor=159.688

The [pH] unit is unique in the sense that it is the only unit defined in UCUM that is a special unit and defined on the mole unit. So if mole <-> mass conversion is enabled, in theory it should be possible to also convert [pH] to a mass unit. But it is unclear what this even means or in which context it would be used. Therefore, ucumate explicitly does not allow the conversion of [pH] to a mass unit.

// Assuming ucumate.enableMolMassConversion=true
ucumService.convert("1", "[pH]", "g", "18.01528"); // yields a TermContainsPHAndCanonicalizingToMass failure result
ucumService.convert("1", "[pH]", "/l"); // yields a Success

Caching or storing any canonicalization/conversion where the mol unit (or any unit defined on mol) is not supported. Caching these calculations would require the provided substance's molar mass coefficient to be stored in the cache key as well which is not implemented at the moment.