Problem Set 11 Solutions

128. Why do people who live at high altitudes add salt to water when boiling foods like pasta? Determine themolality of NaCl required to raise the boiling point of water by 3ºC. Assume that K b for water is0.51ºC/molal. Does adding about 1 teaspoon of NaCl to four quarts of water substantially change theboiling point?The equation for boiling point elevation isΔT b = K b m solute .In order to calculate the molality of NaCl required to raise the boiling point of water by 3ºC, we must solve forthe molality and substitute €m solute = ΔT bK bNote that this is the total molality of the solute particles required. Since for each molecule of NaCl, twoparticles are released in solution (an € Na + ion and a Cl – ion), the molality of NaCl required is really half theamount calculated, or=3 ! C0.51 ! C/mm solute = 5.88 molal.m NaCl = 2.94 molal.How much NaCl is this for four quarts of water? Four quarts of water can be converted to liters, usingconversion factors from the CRC, €"4 Quarts 0.946 L %$ ' = 3.784 L .# 1Quart &Using the density of water as 1.0 g/mL, 3.784 L of water weighs about 3784 g or 3.784 kg. The molality ofNaCl desired to raise the boiling € point of water by 3ºC is 2.94 molal = 2.94 moles of NaCl per kg of water, sofor 3.784 kg of water, this would be 11.125 moles of NaCl. The amount of NaCl in grams required to raise theboiling point of water by 3ºC is"11.125moles 58.44 g %$ ' = 650 g NaCl .# mol &650 grams is much more than 1 teaspoon; 650 g is about 1.5 pounds of NaCl! Thus, the teaspoon of salt addedto boiling water is not really € added to raise the boiling point; it is just for taste.