The percentage of elements
Reminder
The atomic (molar) mass of an element ($ M $) is the mass (in grams) of one mole of atoms of this element!
The atomic (molar) masses are found in the → periodic table
The molar mass of a substance ($ M $) is the mass (in grams) of one mole of molecules or structural units of that substance!
The molar masses of substances are calculated by summing the atomic masses of the → periodic table .
Calculation of the percentage of an element in a substance from the formula of the substance
1) Calculate the molar mass $ M $
2) Calculate the sum $ S $ of the atomic masses of this element (eg if the element intervenes with the coefficient 3, multiply its atomic mass by 3)
3)
$ \% = \frac{S \cdot 100}{M} $

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:

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