Molar mass constant
= Molar mass constant
{title2=$M_{\rm u}=N_{\mathrm A}m_{\rm u}$}
The <molar mass constant> converts <relative atomic mass> or <relative molecular mass> into <molar mass>. It is approximately $1\,\mathrm{g\,mol^{-1}}$. Its <SI> numerical value is determined experimentally: the exact <Avogadro constant> multiplies an experimentally determined <unified atomic mass unit> in kilograms. Treating $M_{\rm u}$ as exactly $1\,\mathrm{g\,mol^{-1}}$ is therefore an excellent elementary-calculation approximation, rather than a defining equality of the modern <SI>.<The International System of Units (SI), ninth edition, SI Brochure>[§2.3.1, The mole, printed p. 131: carbon-12 molar mass no longer exact; M_u = N_A m_u and M(X) = A_r(X) M_u]{ref}