Question

The fully hydrated form of sodium sulfate is the decahydrate Na2SO_4 • 10 H_2O. This compound dehydrates (loses some water of hydration) when heated. A partially dehydrated sodium sulfate sample was found to have a molar mass of 323.1 g/mol. How many water molecules are found per formula unit in this sample? (Hint: determine n in Na2SO_4 • n H_2O)

Answer 1

To find the number of water molecules in the original sample of sodium sulfate, subtract the final mass from the initial mass to find the mass of water. Convert the mass of water to moles, and then multiply by Avogadro's number and the number of water molecules per formula unit to find the number of water molecules.

Step-by-step explanation:

To determine the number of water molecules in the original sample, we need to find the difference in mass before and after the water evaporates. The mass of the water can be calculated by subtracting the final mass from the initial mass. In this case, the initial mass is 5.0 g and the final mass is 2.6 g, so the mass of water is 5.0 g - 2.6 g = 2.4 g.

To convert grams of water to moles of water, we can use the molar mass of water, which is 18.015 g/mol. So, the number of moles of water is 2.4 g / 18.015 g/mol = 0.133 mol.

Since there are 10 water molecules per formula unit, we can multiply the number of moles of water by Avogadro's number (6.022 x 10^23 molecules/mol) and then multiply by 10 to find the number of water molecules per formula unit. So, the number of water molecules per formula unit is 0.133 mol x 6.022 x 10^23 molecules/mol x 10 = 8.03 x 10^23 molecules.