Calculate the percentage of hydrogen in water. First, the molar mass $ M $ of the water:

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$M$ $=$ $18\frac{g}{mol}$

Then the sum $ S $ of the atomic masses of hydrogen:

$S$ $=$ $ 2\cdot M_{H}$ $=$ $2\frac{g}{mol}$

Then the percentage of hydrogen:

$\%_{H}$ $=$ $\frac{S\cdot 100}{M}$ $=$ $11.1$