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Practice each question, then open the answer or worked solution.
With regard to the gaseous state of matter which of the following statements
are correct?
(i) Complete order of molecules
(ii) Complete disorder of molecules
(iii) Random motion of molecules
(iv) Fixed position of molecules
Option (ii) and (iii) are the answers.
Which of the following figures does not represent 1 mole of dioxygen gas at
STP?
(i) 16 grams of gas
(ii) 22.7 litres of gas
(iii) 6.022 × 1023 dioxygen molecules
(iv) 11.2 litres of gas
Option (i) and (iv) are the answers.
Under which of the following two conditions applied together, a gas deviates
most from the ideal behaviour?
(i) Low pressure
(ii) High pressure
(iii) Low temperature
(iv) High temperature
Option (ii) and (iii) are the answers.
Which of the following changes decrease the vapour pressure of water kept in
a sealed vessel?
(i) Decreasing the quantity of water
(ii) Adding salt to the water
(iii) Decreasing the volume of the vessel to one-half
(iv) Decreasing the temperature of the water
Option (ii) and (iv) are the answers.
III. Short Answer Type
If 1 gram of each of the following gases are taken at STP, which of the gases
will occupy (a) greatest volume and (b) smallest volume?
CO, H2O, CH4, NO
(a) CH4 has the least molar mass (16gm), so it would occupy the highest volume for 1gm of the gas.
(b) Similarly, the higher the molar mass, the lesser is the volume occupied. NO has the highest molar mass (30gm), so it would occupy the least volume for 1gm of the gas.
Physical properties of ice, water and steam are very different. What is the chemical composition of water in all three states?
H2O exists in the solid form as ice, it exists in the liquid form as water and as steam in the gas state. All of these states consist of H2O, due to which no change in the chemical composition of water is observed in these states.
The behaviour of matter in different states is governed by various physical laws. According to you what are the factors that determine the state of matter?
The factors that determine the states of matter can be determined by 2 laws.
Charles Law
It states that when pressure is kept constant, the volume of an ideal gas (V) is directly proportional to the absolute temperature (T).
V1/T1 = V2/T2
Boyle’s Law.
It states that when the temperature is kept constant, the pressure of the given mass of an ideal gas (P) is inversely proportional to the volume(V).
P1V1 = P2V2
If we combine two laws,
P1V1/T1 = P2V2/T2
So we get to know that pressure, volume and temperature along with mass are the factors which determine the state of matter.
Use the information and data given below to answer the questions (a) to (c):
• Stronger intermolecular forces result in a higher boiling point.
• Strength of London forces increases with the number of electrons in the
molecule.
• Boiling point of HF, HCl, HBr and HI is 293 K, 189 K, 206 K and
238 K respectively.
(a) Which type of intermolecular forces are present in the molecules
HF, HCl, HBr and HI?
(b) Looking at the trend of boiling points of HCl, HBr and HI, explain
out of dipole-dipole interaction and London interaction, which one
is predominant here.
(c) Why is the boiling point of hydrogen fluoride highest while that of
hydrogen chloride lowest?
(a) Since the halides are a polar molecule, due to the presence of permanent dipoles, the dipole-dipole interactions along with the London forces are found in these halides (HF, HCl, HBr).
(b) The boiling point increases from HI to HCl (as I- is a very large ion, it is bonded by Van der Waal’s forces, so more energy is required to break the bonds. As the size of the ions decreases, lesser energy is required). From this trend, we can conclude that London’s forces are predominant because these interactions also increase from HF to HI.
(c) In HCl, size of Cl- ion is very small, due to which the bonded Van der Waal’s forces are very weak, so less energy is required to break the bonds. Thus the Boiling point is the least.
What will be the molar volume of nitrogen and argon at 273.15K and 1 atm?
We know that 1 mole of any gases at STP (273K, 1 atm) occupies a volume of 22.4L.
The molar volume (volume of 1 mole of the gas), therefore these gases occupy 22.4L of volume.
A gas that follows Boyle’s law, Charles law and Avogadro’s law are called an ideal gas. Under what conditions a real gas would behave ideally?
All gases are not ideal gas. Real gas doesn’t obey the gas law at normal temperature and pressure condition. A gas behaves ideally under two conditions these are – (i) the volume of the molecule of a gas is negligible as compared to its complete volume. (ii) There is no attraction force between the gases.