The correct answer is A kilogram of hydrogen.
The atomic mass of hydrogen is 1 g/mole. To make up 1 kilogram or 1000 g of hydrogen element, 1000 moles of hydrogen are required. Each mole has an Avogadro's number of atoms or 6.023 xx 10^(23) atoms. Thus, 1 kg hydrogen element or 1000 moles of hydrogen have 6.023 xx 10^(26) atoms (or 1000 times the Avogadro's number).
In the one-kilogram iron weight, there are 1.08·1025 atoms, which corresponds to the chemical amount of approximately 17.9 moles of atoms.
The atomic number of iron is 55.847. Therefore a mole of iron, which contains 6.022×10^23 iron atoms, has a mass of 55.847 g, and 25 g of iron contains 25×6.022×10^23/55.847 iron atoms.
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