Which of the following statements is false ? The viscosity of a liquid decreases
ID: 877043 • Letter: W
Question
Which of the following statements is false?
The viscosity of a liquid decreases as temperature increases.
The vapor pressure of a substance increases as temperature increases.
The vapor pressure of a substance with strong intermolecular forces is greater than that of a substance with weak intermolecular forces (at the same temperature).
A substance boils when its vapor pressure is equal to the atmospheric pressure.
The boiling point of water increases as atmospheric pressure increases.
2.
The half-life for a reaction that is first-order in A is 15 minutes. How long will it take for the concentration of A to change from 0.16 M to 0.040 M?
A(g) B(g) + C(g)
60 min
45 min
30 min
23 min
15 min
The viscosity of a liquid decreases as temperature increases.
The vapor pressure of a substance increases as temperature increases.
The vapor pressure of a substance with strong intermolecular forces is greater than that of a substance with weak intermolecular forces (at the same temperature).
A substance boils when its vapor pressure is equal to the atmospheric pressure.
The boiling point of water increases as atmospheric pressure increases.
2.
The half-life for a reaction that is first-order in A is 15 minutes. How long will it take for the concentration of A to change from 0.16 M to 0.040 M?
A(g) B(g) + C(g)
60 min
45 min
30 min
23 min
15 min
Explanation / Answer
1. The statement which is false is : The vapor pressure of a substance with strong intermolecular forces is greater than that of a substance with weak intermolecular forces (at the same temperature).
Reason: Every substance also has an associated vapor pressure with it. The vapor pressure is defined to be the amount of gas of a compound that is in equilibrium with the liquid or solid. If the intermolecular forces are weak, then molecules can break out of the solid or liquid more easily into the gas phase. Consider two different liquids, one polar one not, contained in two separate boxes. We would expect the molecules to more easily break away from the bulk for the non-polar case. This would mean that, proportionately, there are more molecules in the gas phase for the non-polar liquid. This would increase the vapor pressure. Thus, unlike the physical properties listed above, the vapor pressure of a substance decreases with increasing intermolecular forces.
All the rest statements are true.
2. For first order reactions,
t(1/2) = 0.693/k
k = 0.693/ [t(1/2)]
k = 0.693 / (15 x 60 seconds) = 7.7E-4
now,
ln[A]ln[A]o = kt ( [A] = conc of A at time t and [A]o = conc of A at time 0 sec)
ln(0.04M) - ln(0.16M) = -(7.7E-4)t
-3.2 - (-1.8) = -7.7E-4 x t
t = 1800seconds = 30 minutes
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