Atom weight in SI units

Question

In chemistry is very common to use the units grams/mol for the atomic weight, so the mass of one atom (of Tin, for example) is calculated as :

massOfOneAtom = (0.11871 (Kg/mol))/(AvogadroConstant (1/mol))= 1.97123*10^-25 Kg

When I try to do that with Mathematica, the Output is:

ma = ElementData["Tin", "AtomicWeight"]
Na = Quantity["AvogadroConstant"]

massOfOneAtom = ma/Na

118.710u

1 Avogadro constant

118.710 u/Avogadro constant

How can I express this calculation to have a numeric result of the mass of one atom in standard SI units?

Answer 1

Since you ask "How can I express this calculation to have a numeric result ..."

myQ = Quantity["AvogadroConstant"] Quantity[1, "Mol"]

6.022141*10^23

myM = QuantityMagnitude[ElementData["Tin", "AtomicWeight"]]

118.710

myM/myQ

1.97123*10^-22

Have a test on Board:

UnitConvert[ElementData["Tin", "AtomicWeight"], "Grams"]

1.97123*10^-22 g

Answer 2

To calculate the mass of one tin atom as the OP suggests, it is probably more convenient to obtain the Molar Mass, and then divide by the Avogadro constant.

ElementData["Tin", "MolarMass"] / Quantity["AvogadroConstant"] // UnitConvert

1.97123 × 10^-25 kg

The default output for MolarMass is (conveniently) g.mol^-1 (rather than Kg.mol^-1)

ElementData["Tin", "MolarMass"]

118.710g/mol

Mathematica (apparently) makes no distinction between AtomicWeight and AtomicMass (IUPAC discussion), (the default output (v11) is unified atomic mass units, u), and the result the OP requires may also be obtained as follows:

ElementData["Tin", #] & /@ {"AtomicWeight",  "AtomicMass"} // UnitConvert

{1.97123 × 10^-25 kg, 1.97123 × 10^-25 kg}

Although the default output for MolarMass is g.mol^-1, Mathematica is clever enough to evaluate the following as true

(ElementData["Tin", "MolarMass"]/Quantity["AvogadroConstant"]) == 
ElementData["Tin", "AtomicWeight"]

True