Question

Surface tension accounts for which of the following observations?

a) liquids can be used in hydraulic brakes

b) a drop of ink diffuses slowly through water

c) water has a definite vapor pressure at 25°C

d) the top of a column of liquid mercury is rounded instead of flat

e) water assumes the shape of its container

Which of the following aqueous solutions would have the highest boiling point?

a) 1.0 mole of Na3PO4 in 1.0 kg of water

b) 1.0 mole of Ca3(PO4)2 in 1.0 kg of water

c) 2.0 moles of AlPO4 in 1.0 kg of water

d) 1.0 mole of Na2S in 1.0 kg of water

e) 2.0 mole of NaCl in 1.0 kg of water

Answer 1

Surface Tension is a property of liquids that make them aquire the least surface area possible. The dome (rounded) shape at the top of the mercury column gives less surface area to the entire mass of mercury than that if the surface were flat. Therefore surface tension accounts for

d) the top of a column of liquid mercury is rounded instead of flat

Elevation in boiling point = i x Kb x m

where,

i = vant hoff factor

Kb = boiling point elevation constant = 0.512 degree C / m = Constant for all

m = molality of solution

Since Kb is constant we will calculate the product (i x m) to check for the elevation in boiling points of the above solutions

a) 1.0 mole of Na3PO4 in 1.0 kg of water (i = 4, m = 1) => (i x m) = 4

b) 1.0 mole of Ca3(PO4)2 in 1.0 kg of water (i = 5, m = 1) = (i x m) = 5 (Since i x m is maximum for this solution this will have the maximum boiling point)

c) 2.0 moles of AlPO4 in 1.0 kg of water (i = 2, m = 2) => (i x m) = 4

d) 1.0 mole of Na2S in 1.0 kg of water (i = 3, m = 1) => (i x m) = 3

e) 2.0 mole of NaCl in 1.0 kg of water (i = 2, m = 2) => (i x m) = 2