# Calculate the mass of hydrogen in 90 g of water.

Answer: m (H) = 10 g.
Explanation of the solution to the problem: to solve this problem, you must first find the mass fraction of hydrogen in water, for this you need to use the following formula:
w = n * Ar / M * 100%, where w is the mass fraction of the component in the compound, n is the number of atoms of the given component, Ar is the atomic mass of the given element, M is the molar mass of the entire compound.
Substituting the known values ​​into the formula, we get:
w (H) = n * Ar / M * 100% = 2 * 1/18 * 100% = 0.11.
The last step is to find the mass of hydrogen in 90 g of water, for this we multiply the mass fraction of hydrogen in water by the mass of water: m (H) = w (H) * m (H2O) = 0.11 * 90 = 10 g.

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