Avogadro's constant $N_\mathrm A$ is defined as the number of constituent particles (usually atoms or molecules) contained in the amount of substance given by one mole.
The value of Avogadro's constant $N_\mathrm A$ is going to be set to exactly $6.02214076\times 10^{23}\ \mathrm{mol}^{-1}$ in 2019.
I might add that the kilogram is also going to be redefined in 2019 based on Planck's constant.
Will these changes result in the molar mass of a substance no longer being exactly equal to the molecular mass expressed in grams?