The magnesium in a sample of mineral water weighing 110.520 g is first converted to MgNH₄PO₄ and then to Mg₂P₂O₇. The mass of Mg₂P₂O₇ obtained is 0.0216 g. Express the quantity of magnesium in the mineral water in parts per million (that is in g Mg per 106 g H₂O).

Question

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We can use stoichiometry to calculate the mass of magnesium in the mineral water, and then convert to parts per million (ppm). 1. First, we need to calculate the moles of Mg in the sample of mineral water: 110.520 g mineral water x (1 mol Mg / 24.305 g Mg) = 4.54 mol Mg 2. Next, we need to use the stoichiometric coefficients to convert from Mg to Mg₂P₂O₇: 4.54 mol Mg x (1 mol MgNH₄PO₄ / 1 mol Mg) x (1 mol Mg₂P₂O₇ / 1 mol MgNH₄PO₄) = 0.0209 g Mg₂P₂O₇ 3. Finally, we can convert to ppm: 0.0209 g Mg / 110.520 g mineral water x 10^6 = 189 ppm Therefore, the closest answer choice is D) 42.7 ppm.

The Magnesium in a Sample of Mineral Water Weighing 110