The enthalpy of vaporization at 298 K for diethylether (C_4H_10O) is 26.0 kJ/mol
ID: 532797 • Letter: T
Question
The enthalpy of vaporization at 298 K for diethylether (C_4H_10O) is 26.0 kJ/mol. How much heat would be required to vaporize 1.00 L of the ether at 298 K if its density is 0.714 g/L? 440 J 250 J 186 J 130 J 74.1 J What is the difference between "normal boiling point and boiling point of a liquid? "Normal boiling point" is a boiling point of a liquid at normal (standard) pressure and temperature, while "boiling point" is measured at any other conditions of pressure and temperature. "Normal boiling point is a boiling point of 1 mol of a liquid with a surface of 1 m^2, while "boiling point" is boiling temperature for any other amount of a substance. "Normal boiling point" is a boiling point a liquid has at 1 atm of pressure:"boiling point" is a boiling temperature of a liquid at any other pressure. "Normal boiling point and "boiling point" are synonyms. "Normal boiling point refers to 1 g of substance:"boiling point" refers to any other amount of substance. How many joules of energy are needed to change 10.0 g of ice at 0.00 degree C to water at 20.0 degree C?Explanation / Answer
Ans. 13. Volume of ether = 1.00 L
Mass of ether = Volume x Density = 1.00 L x (0.714 g/ mL)
= 1000 mL x (0.714 g/ mL)
= 714.00 g
Note: Density of ether = 0.714 g/ mL or, 0.714 kg/ L but NOT 0.714 g/ L
Moles of ether = Mass / Molar mass
= 714.00 g/ (74.1228 g/ mol)
= 9.6327 mol
Now,
Amount of heat required =
(Moles of ether to be vaporized x molar enthalpy of vaporization)
= 9.6327 mol x 26.0 kJ/mol
= 250.45 kJ
= 250 kJ (rounded off to nearest whole number)
Therefore, correct option is B.
#14. Boiling point of a substance is the temperature at which its vapor pressure equates the pressure surrounding the liquid phase. The pressure P is a variable for boiling point, and pressure must be defined.
Normal boiling point is the temperature at which vapor pressure of water is equal to 1 atm pressure (Pressure is fixed to 1 atm).
Option A. Incorrect. Fr normal boiling point, only P is fixed at 1 atm while T may vary depending on the substance. Boiling point can also be measured at 1 atm and stated so.
Option B. Normal boiling point as well as boiling point at specified P are intrinsic property of the substance. For example, normal boiling point of water is 1000C- both when you take 18.0 g (= 1 mol) or any other amount of water. Similarly, water boils irrespective of the surface area- at 1 atm, 1000C when kept in a 250.0 mL conical flask (very small surface area) or a large tub with surface area of 1 m2. Note that rate of evaporation depend on exposed surface area whereas boiling point remains unaffected (it’s an intrinsic property).
Option C. Correct. [see definitions]
Option D. Incorrect [see definitions]
Option. E. Incorrect. [See, B]
#15. When melting the ice from 00C to 20.00C, the energy is required at following two events-
Step 1: All ice is converted into water at 00C (phase transition)-
Energy required, q1 = m x C
Where, m= mass of ice ; C = heat of fusion
Or, q1 = 10.0 g x 335.0 J/g = 3350.0 J
Step 2: Water’s temperature is raised from 00C to 20.00C-
Heat absorbed by water is given by-
q = m x s x dT - equation 2
Where,
q = heat gained
m = mass of water in gram
s = specific heat, [ s = 4.184 J g-10C-1 for water]
dT = change in temperature = Final temperature, T2 – Initial temperature, T1
Putting the values in equation 2-
q2 = 10.0 g x (4.184 J g-10C-1) x (20.00C – 00C)
or, q2 = 836.8 J
Now,
Total heat required = q1 + q2
= 3350.0 J + 836.8 J
= 4186.8 J
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