How many hydrogen atoms are present in 75.0 of g H2O? Given molar mass of water is 18.02 g/mol.

Question

BlackTom AI · Accepted Answer

First, restate the problem to ensure we understand it: We need to find how many hydrogen atoms are present in 75.0 g of H2O, given the molar mass of water is 18.02 g/mol.
Option 1: 7.53 × 10^24 atoms. This value would correspond to about 1.18 × 10^24 mol of H atoms if converted directly, which does not align with the stoichiometry of H2O. Since each mole of water contains 2 moles of H atoms, this option underestimates the proper amount.
Option 2: 75.0 atoms. This is far too small by many orders of magnitude. 75.0 grams contains many moles, and the number of atoms should be on the order of 10^24, not a mere 75.
Option 3: 4.17 atoms. This is nonsensical in chemistry context: any macroscopic sample contains an astronomically large number of atoms; a result of a few atoms would imply an impossible, highly unrealistic calculation error (e.g., attempting to count atoms directly from grams without Avogadro’s number). Therefore incorrect.
Option 4: 5.02 × 10^24 atoms. This matches the correct stoichiometric reasoning: convert grams to moles of H2O, multiply by 2 to get moles of H atoms, then convert to atoms using Avogadro’s number. Step by step: moles H2O = 75.0 g / 18.02 g/mol ≈ 4.16 mol. Each mole of H2O has 2.00 mol of H atoms, so moles of H atoms ≈ 8.32 mol. Atoms = 8.32 mol × 6.022×10^23 atoms/mol ≈ 5.02×10^24 atoms. Therefore this answer is consistent with standard molar-quantity conversions.
Option 5: 2.51 × 10^24 atoms. This is roughly half of the correct value, suggesting a missed factor of 2 somewhere (perhaps forgetting to account for the two hydrogens per water molecule, or a miscalculation in moles). The reasoning would delay in matching the 8.32 mol of H atoms required, so this option is incorrect.
In summary, the correct choice reflects converting grams to moles of water, doubling to account for two hydrogens per molecule, and then converting moles of hydrogen to atoms using Avogadro’s number, yielding about 5.02 × 10^24 atoms.

How many hydrogen atoms are present in 75.0 of g H2O? Given molar mass of water is 18.02 g/mol.单项选择题

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