Dew Point and Humidity

Q.

A bucket full of water is placed in a room at ${15}^0$ C with initial relative humidity 40 %. The volume of the room is 50 $m^3$. (a) How much water will evaporate ? (b) If the room temperature is increased by $5^0$ C, how much more water will evaporate ? The saturation vapour pressure of water at ${15}^0$ C and ${20}^0$ C are 1.6 kPa and 2.4 kPa respectively.

Q.

The temperature and relative humidity in a room are 300 K and 20 % respectively. The volume of the room is 50 ma. The saturation vapour pressure at 300 K is 3-3 kPa. Calculate the mass of the water vapour present in the room.

Q.

On a winter day, the outside temperature is $0^0$ C and relative humidity 40 %. The air from outside comes into a room and is heated to ${20}^0$ C. What is the relative humidity in the room ? The saturation vapour pressure at $0^0$ C is 4.6 mm of mercury and at ${20}^0$ C it is 18 mm of mercury.

Q.

A barometer tube is 80 cm long (above the mercury reservoir). It reads 76 cm on a particular day. A small amount of water is introduced in the tube and the reading drops to 75.4 cm. Find the relative humidity in the space above the mercury column if the saturation vapour pressure at the room temperature is 1.0 cm.

Q. A glass contains some water at room temperature ${20}^{\circ }C$. Refrigerated water is added to it slowly. When the temperature of the glass reaches ${10}^{\circ }C$, small droplets condense on the outer surface. Calculate the relative humidity in the room. The boiling point of water at a pressure of 17.5 mm of mercury is ${20}^{\circ }C$ and at 8.9 mm of mercury it is ${10}^{\circ }C$.

1. $60\%$
2. $51\%$
3. $35\%$
4. $75\%$

Q.

The temperature and the relative humidity are 300 K and 20 % in a room of volume 50 m a. The floor is washed with water, 500 g of water sticking on the floor. Assuming no communication with the surrounding, find the relative humidity when the floor dries. The changes in temperature and pressure may be neglected. Saturation vapour pressure at 300 K = 3.3 kPa.

Q.

50 m of saturated vapour is cooled down from ${30}^0$ C to ${20}^0$ C. Find the mass of the water condensed. The absolute humidity of saturated water vapour is 30 $g{m}^{-3}$ a at ${30}^0$C and 16 $g{m}^{-3}$ at ${20}^0$ C.

Q. The weather report reads,
“Temperature: $20{}^{\circ }C$,
Relative humidity: $100\%$”.
What is the dew point?

1. Less than ${20}^{\circ }C$
2. ${20}^{\circ }C$
3. More than ${20}^{\circ }C$
4. Can’t be said.

Q.

The temperature and the dew point in an open room are ${20}^0$C and ${10}^0$C. If the room temperature drops to ${15}^0$C, what will be the new dew point ?

Q.

A glass contains some water at room temperature ${20}^0$C. Refrigerated water is added to it slowly. When the temperature of the glass reaches ${10}^0$ C, small droplets condense on the outer surface. Calculate the relative humidity in the room. The boiling point of water at a pressure of 17.5 mm of mercury is ${20}^0$ C and at 8.9 mm of mercury it is ${10}^0$ C.

Q. The temperature and the dew point in an open room are $20{}^{\circ }C$ and $10{}^{\circ }C$. If the room temperature drops to $15{}^{\circ }C$, what will be the new dew point?

1. ${10}^{\circ }C$
2. ${11}^{\circ }C$
3. ${15}^{\circ }C$
4. ${20}^{\circ }C$

Q. In a weather report the temperature is given as ${20}^{o}C$ and dew point ${10}^{o}C$. The S.V.P at these temperatures are respectively $17mm$ and $9mm$ of mercury. What is the relative humidity?

1. $50\%$
2. $91\%$
3. $53\%$
4. $60\%$

Q.

The weather report reads, "Temperature 20^0 C : Relative humidity 100 %". What is the dew point ?

Q. A rubber ball floats in water with $2/7$ of its volume above the surface of water$.$ Calculate the average relative density of rubber ball$.$

Q. $200g$ of hot water at ${80}^{o}C$ is added to $300g$ of cold water at ${10}^{o}C$. Neglecting the heat taken by the container, calculate the final temperature of the mixture of water. Specific heat capacity of water $=4200Jk{g}^{-1}{K}^{-1}$

1. ${48}^o$ C
2. ${18}^o$ C
3. ${28}^o$ C
4. ${38}^o$ C

Q. Dew point can be reached by increasing the temperature of the system.

1. True
2. False

Q. In some areas, people sweat a lot because the relative humidity is higher as compared to other areas.

1. True
2. False

Q. A block of wood weights 12 kg and has a relative density 0.6. It is to be of its volume immersed. What weight of a metal is needed if the metal is on the top of wood? [Relative density of metal = 14].

1. 2 kg
2. 4 kg
3. 6 kg
4. 8 kg

Q. Dew point can be reached by increasing the temperature of the system.

1. True
2. False

Q. The temperature and relative humidity in a room are $300K$ and $20\%$ respectively. The volume of the room is $50m^3$. The saturation vapour pressure at $300K$ is $3.3kPa$. Calculate the mass of the water vapour present in the room.

Q. Quantity of heat lost in condensation of $10gm$ of steam at ${100}^{\circ }C$ is

1. $2.26\times {10}^{5}J$
2. $2.26\times {10}^{4}J$
3. $22.6J$
4. $44.52\times {10}^{4}J$

Q.

The speed of molecules at absolute zero temperature becomes

1. zero.
2. infinity.
3. less.
4. more.

Q. If there are no heat losses, the heat released by the condensation of x g of steam at ${100}^{o}C$ into water at ${100}^{o}C$ can be used to convert y g of ice at $0^{o}C$ into water at ${100}^{o}C$. Than ratio y x is nearly

1. $1:1$
2. $2:1$
3. $3:1$
4. $2.5:1$

Q. Mercury is used as a barometric liquid because it is ______ times denser than water

1. $2$
2. $13.6$
3. $10.34$
4. $5$