chemistry : class11 Experimental Measurement of Surface Tension : Surface tension : States of Matter: Gases and Liquids MCQ Questions and Answers
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Experimental Measurement of Surface Tension
MCQ questions and answers.
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MCQ Questions
1. In a capillary-rise experiment, water of density 800 kg m^-3 rises by 3.06 cm in a capillary tube of radius 0.50 mm. Assuming complete wetting and g = 10 m s^-2, what is the surface tension of water?
A.
0.0245 N m^-1
B.
0.0612 N m^-1
C.
0.1224 N m^-1
D.
0.612 N m^-1
Correct Answer:
0.0612 N m^-1
Explanation:
For capillary rise, gamma = rho g r h divided by 2 cos theta. For complete wetting, cos theta = 1. Thus gamma = (800)(10)(0.00050)(0.0306)/2 = 0.0612 N m^-1.
2. A liquid rises to heights 4.0 cm and 2.5 cm in two capillary tubes of radii 0.20 mm and 0.32 mm, respectively. If the contact angle is the same in both tubes, which conclusion is correct about the validity of the capillary-rise relation?
A.
The product rh is approximately constant, supporting the relation
B.
The ratio h/r is approximately constant, supporting the relation
C.
The sum r + h must be constant for capillary rise
D.
The product r divided by h must be constant for capillary rise
Correct Answer:
The product rh is approximately constant, supporting the relation
Explanation:
For the same liquid and contact angle, gamma = rho g r h divided by 2 cos theta, so rh must remain constant. Here, 0.20 x 4.0 = 0.80 and 0.32 x 2.5 = 0.80 in consistent units.
3. A liquid of density 900 kg m^-3 rises 2.0 cm in a capillary tube of radius 0.40 mm. If its surface tension is 0.045 N m^-1 and g = 10 m s^-2, what is the cosine of its contact angle with the capillary material?
A.
0.25
B.
0.40
C.
0.50
D.
0.80
Correct Answer:
0.40
Explanation:
Using gamma = rho g r h divided by 2 cos theta, cos theta = rho g r h divided by 2 gamma. Substitution gives cos theta = (900)(10)(0.00040)(0.020)/(2 x 0.045) = 0.40.
4. In a drop-weight experiment, a drop detaches from a tube of radius 1.0 mm when its mass is 3.14 mg. Taking g = 9.8 m s^-2 and neglecting the correction factor, the surface tension is approximately:
A.
0.0049 N m^-1
B.
0.0154 N m^-1
C.
0.0490 N m^-1
D.
0.154 N m^-1
Correct Answer:
0.0049 N m^-1
Explanation:
At detachment, mg = 2 pi r gamma for a circular tube. Therefore gamma = mg divided by 2 pi r = (3.14 x 10^-6 x 9.8)/(2 pi x 10^-3) = 0.0049 N m^-1.
5. Equal volumes of two liquids are allowed to form drops from the same stalagmometer. The first liquid gives 48 drops and has density 1.20 g cm^-3; the second gives 60 drops and has density 0.80 g cm^-3. If the surface tension of the first liquid is 0.072 N m^-1, what is that of the second liquid?
A.
0.0384 N m^-1
B.
0.0576 N m^-1
C.
0.0900 N m^-1
D.
0.108 N m^-1
Correct Answer:
0.0576 N m^-1
Explanation:
For equal volume discharged, gamma is proportional to density divided by the number of drops. Hence gamma2 divided by gamma1 = (rho2 n1)/(rho1 n2) = (0.80 x 48)/(1.20 x 60) = 0.5333. Therefore gamma2 = 0.072 x 0.5333 = 0.0576 N m^-1.
6. A stalagmometer is calibrated using water. The same volume between two marks gives 50 drops of water and 75 drops of an unknown liquid. The densities of water and the unknown liquid are 1.00 g cm^-3 and 0.80 g cm^-3, respectively. If surface tension of water is 0.072 N m^-1, what is the surface tension of the unknown liquid?
A.
0.0320 N m^-1
B.
0.0384 N m^-1
C.
0.0480 N m^-1
D.
0.0675 N m^-1
Correct Answer:
0.0384 N m^-1
Explanation:
For equal discharged volume, gamma_unknown divided by gamma_water = (rho_unknown n_water)/(rho_water n_unknown). Thus gamma_unknown = 0.072 x (0.80 x 50)/(1.00 x 75) = 0.0384 N m^-1.
7. In a film-balance experiment, a liquid film pulls a movable wire of length 5.0 cm with a force of 14.0 mN. If the film has two surfaces and end effects are neglected, its surface tension is:
A.
0.014 N m^-1
B.
0.070 N m^-1
C.
0.140 N m^-1
D.
0.280 N m^-1
Correct Answer:
0.140 N m^-1
Explanation:
A liquid film has two surfaces, so F = 2 gamma L. Therefore gamma = F divided by 2L = 0.014/(2 x 0.050) = 0.140 N m^-1.
8. In a capillary-rise measurement, the radius of the tube is measured 2% too high and the height of rise is measured 3% too low. All other quantities are correct. What is the approximate percentage error in the calculated surface tension?
A.
1% too high
B.
1% too low
C.
5% too high
D.
5% too low
Correct Answer:
1% too low
Explanation:
Since gamma is proportional to rh, the measured value changes by factors 1.02 and 0.97. Their product is 0.9894, so the calculated surface tension is approximately 1% lower than the true value.
9. A capillary-rise experiment gives a rise of 5.0 cm for a liquid of density 1000 kg m^-3 in a tube of radius 0.30 mm. If the liquid does not completely wet the tube and its contact angle is 60 degrees, with g = 9.8 m s^-2, the measured surface tension is approximately:
A.
0.0368 N m^-1
B.
0.0735 N m^-1
C.
0.147 N m^-1
D.
0.294 N m^-1
Correct Answer:
0.147 N m^-1
Explanation:
The capillary relation is gamma = rho g r h divided by 2 cos theta. With cos 60 degrees = 0.5, gamma = (1000)(9.8)(0.00030)(0.050)/(2 x 0.5) = 0.147 N m^-1.
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