Question

Consider a sample of helium and a sample of neon, both at ${{30}^{\circ }}C$ and 1.5 atm. Both samples have a volume of 5 L. Which statement concerning these samples is not true?
[A] Each sample contains the same number of atoms of gas
[B] Each sample weighs the same amount
[C] Each sample contains the same number of moles of gas
[D] The density of neon is greater than the density of helium

Answer 1

HINT: To answer this, use the equation, PV = n R T. Compare the number of moles for each sample and using it you can find the mass as well as the density. You can use this to find the correct answer to this question.

COMPLETE STEP BY STEP SOLUTION: We know that both helium and neon are noble gases. Here, the temperature, pressure and volume are given to us as ${{30}^{\circ }}C$,1.5 atm and 5 L respectively. Now, let us discuss the options one by one and figure out the correct one. Firstly, we have that each sample contains the same number of atoms of gas. We know that at same temperature and pressure, equal volumes of two gases contain the same number of molecules regardless of their chemical structure and formula. Therefore, this option is correct. In the third option, we have that each sample contains the same number of moles of gas. According to the ideal gas equation, we know that PV = nRT. Since the temperature, pressure and volume are the same for both the gases thus if we calculate for n, we will get the same value for both the gases. Therefore, this option is also correct. In the second option, we have each sample weigh the same amount. We know that we can calculate the mass from the number of moles of gas and its molar mass. Here, we have the same number of moles of both the gases. However, to get equal masses, their molar masses must also be the same. The molar mass of helium is 4 and that of neon is 20. Thus, their weight cannot be the same. Therefore, this option is incorrect. And lastly, we have that the density of neon is greater than the density of helium. We have already seen above that the molar mass of neon is greater and thus its mass will also be higher. Density is directly proportional to mass. So, this option is also correct. We can understand from the above discussion that the incorrect statement about the given samples is that they will weigh the same.

So, the correct answer is option [B] Each sample weighs the same amount.

NOTE: Here, helium and neon both are noble gases. Noble gases are a class of chemical elements having similar properties. They are placed in the group 18 in the periodic table and they are considered as inert elements. They do not react readily due to their stable configuration and under standard conditions they are odourless, colourless and have a low chemical reactivity.