Avogadro's constant $N_A$ 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 $N_A$ is going to be set to exactly $6.02214076\times 10^{23}\text{mol}^{-1}$ 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?

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    $\begingroup$ related chemistry.stackexchange.com/questions/41273/… $\endgroup$ – Mithoron Nov 17 '18 at 20:33
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    $\begingroup$ Atomic weights will no longer be based on carbon 12. But unless you're experimenting to 1 part per billion or better you won't notice the difference. $\endgroup$ – MaxW Nov 17 '18 at 21:01

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