In determining the mass percent in CH2O. We have to multiply the molar mass of hydrogen by 2 because:

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When evaluating why we multiply the hydrogen mass by 2 in CH2O, we start by identifying what the molecule actually contains.
Option 1: "because there is a carbon and an oxygen" — This is not the reason. The presence of C and O does not dictate multiplying H by 2; mass percent depends on the number of hydrogen atoms, not on which other elements are present.
Option 2: "because you always multiply hydrogen's mass by 2" — This is a generalization that isn’t true. We do not multiply hydrogen by 2 in all compounds; we multiply by 2 only when there are two hydrogen atoms in the specific formula unit, as with CH2O.
Option 3: "there are two hydrogens" — This is correct. CH2O contains two hydrogen atoms, so the total mass contributed by hydrogen is 2 × (mass of one H). This directly affects the hydrogen portion of the molar mass and thus the mass percent of hydrogen in the compound.
Putting it together, to compute mass percent of H in CH2O you take 2 × (molar mass of H) divided by the molar mass of CH2O and multiply by 100%. The key point is counting how many hydrogen atoms are present, not applying a blanket rule or the presence of other elements.
In summary, option 3 correctly identifies the reason—the molecule has two hydrogen atoms—while options 1 and 2 rely on incorrect justifications or overgeneralizations that do not apply here.

1A W25 Emperical and Molecular formulas and Mass Percent and Molecular Composition (3 videos)

In determining the mass percent in CH2O. We have to multiply the molar mass of hydrogen by 2 because:

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