Remember that your desiccant needs to be better than the standard calcium chlori
ID: 493958 • Letter: R
Question
Remember that your desiccant needs to be better than the standard calcium chloride. First, you need to find out how effective calcium chloride is. You know that calcium chloride commonly forms a hexahydrate, i.e. each mole of CaCl_2 can pick up six moles of water molecules. What percentage of its own weight can calcium chloride anhydride absorb in water? One desiccant you plan to test is sodium metasilicate (Na_2SiO_3). What mass of Water will the hydrated sodium metasilicate need to give off per mole to be a better desiccant than calcium chloride? Look up the SDS for sodium metasilicate and determine the most common hydrate form of the compound. Do we need to be concerned about the hydrate form catching on fire while we heat it? Which section gives us this information?Explanation / Answer
SOLUTION:
Q3. Molar mass of CaCl2 = 110.0g
Each mole of CaCl2 binds with 6 moles of H2O.
Mass of 6 moles of H2O = 6 X 18 = 108.
Percentage of H2O w.r.t. mass of CaCl2 = (108/110) X 100 = 98.2%
Q4. To be a better desicant Na2SiO3 must absorb more than 98.2% of water w.r.t. its own mass.
Molar Mass of Na2SiO3 = 122g
Mass of water absorbed if Na2SiO3 absorbs same water as CaCl2 = 122 X 98.2 /100 = 119.8g
Hence Na2SiO3 will have same efficiency as desicant as CaCl2 if one mole of NaSiO3 absorbs 119.8g of water. To be more efficient Na2SiO3 must absorb more than 119.8g. Thus one mole of Na2SiO3 must absorbs 7 moles of water (or 126g of water) to be better desiccant.
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