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Experiment 3 Determination of Waters of Hydration

Experiment 3 Determination of Waters of Hydration. Hydrates are salts that have water present in the structure of their crystal. This water is not tightly bound in the crystal and can be driven off with heat. A hydrate has a general formula of: C x A y · nH 2 O

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Experiment 3 Determination of Waters of Hydration

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  1. Experiment 3Determination of Waters of Hydration Hydrates are salts that have water present in the structure of their crystal. This water is not tightly bound in the crystal and can be driven off with heat. A hydrate has a general formula of: CxAy·nH2O where C is the cation, x is the moles of C, A is the anion, y is the moles of A, and n is the moles of H2O. An example from this lab is CoCl2 ·6H2O. Co is the cation, Cl is the anion, and there are 6 waters of hydration. In this example, x is one and y is two. We will be determining the waters of hydration using a Bunsen burner to evaporate the water out of the hydrate. Bunsen burners produce large amounts of heat powered by natural gas, so precautions must be taken to avoid injury. The steps for using a Bunsen burner follow on the next page.

  2. Connect the Bunsen Burner to the natural gas outlet. Make sure that the air inlet and gas inlet on the Burner are closed finger-tight. Turn the gas outlet valve on all the way. Light a match and hold it at the top of the burner. Slowly open the gas inlet valve until a flame is ignited. Adjust the gas inlet until there is approximately a six inch flame. Open the air inlet valve until the flame is blue. To shut it down, close the gas inlet before the closing the outlet. Never reach around a lit burner! To determine the mole ratio of water in the hydrate, we will measure the mass of the compound before and after heating. The heating will take place in a crucible, so we need to take the crucible weight into account. Say we have an unknown hydrate, MgSO4 ·nH2O. The initial mass of just the hydrate is 3.0719g. After heating, the mass of the remaining salt is 1.5012g. Since water was the only thing driven off, we can find the mass of it: Mass of water = 3.0719g - 1.5012g = 1.5707g To find the mole ratio, we need to find the absolute number of moles for both the salt and the water. To do this, we divide the mass by the molar mass. (1.5012g of MgSO4)/(120.97g/mol MgSO4) = 0.012472 moles of MgSO4 Next, we do the same for the water. (1.5707g of H2O)/(18.02g/mol H2O) = 0.08716 moles of H2O Finally, to find the mole ratio of water to the salt, we divide the moles of water by the moles of salt. (0.08716 mol H2O)/(0.012472 mol MgSO4) = 6.9888 mol H2O/mol MgSO4 Because there is not going to be a fraction of a mole of water, we round up to give us the final formula of the hydrate, MgSO4 ·7H2O. The name of this is magnesium sulfate heptahydrate.

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