Well some back of the napkin math, atomic mass of platinum, the heavier constituent of the standard is 195. So more than 5 mols of these atoms are in the block,

Well some back of the napkin math, atomic mass of platinum, the heavier constituent of the standard is 195. So more than 5 mols of these atoms are in the block, because 1 mol of pt is 195 grams. So there’s around 5610^23 atoms. So 5 parts per 100 million adds up to a lot. Maybe 1.5*10^15 atoms? It’s been a while since I had chemistry...