Let the amount of matter be ν, m is the mass of water, M is the molar mass of water. We know that v = m / M, the molar mass of water = 1 * 2 + 16 = 18 g / mol, because (1 is the mass of a hydrogen atom, 16 is the mass of a hydrogen atom), m = 4.5 g by condition. Substitute the numerical values into the formula and we get that v = 4.5 g / 18 g / mol = 0.25 mol. This means that 4.5 g of water contains 0.25 mol of a substance.
Answer: 0.25 mol.
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