Determine the grams of ethylene glycol (C2H4(OH)2) which must be dissolved in 2500 g of water to make a 4.78m?
Answer:
molality = moles solute / Kg solvent
mass solvent = 2500 g = 2.500 Kg
moles solute = molality x Kg solvent = 4.78 x 2.500 = 11.9
Molar mass ethylene glycol = 62 g/mol
11.9 moles x 62 g/mol = 737.8 g needed
For a molarity of 4.78 there must be 4.78 moles per liter.
Mw = 2* C + 6* H + 2* O = 2* 12 + 6*1 + 2 * 16 = 92
+4.78 * 92 = 439.76 grams/liter
2500 grams = 2.5 liter
2.5 * 439.76 grams = 1099.4 grams, estimate
The reason this is an estimate is that we have not accounted for the change of volume the ethylene glycol makes.
The correct way to make a 4.78 M solution is to put 1099.4 g of ethylene glycol and then fill up the remaining 2.5 liter of volume with water.
Be careful not to confuse molality(m) with molarity(M). Dr. A has the correct answer.
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