Consider a sample of helium and a sample of neon, both at 30ºC and 1.5atm. Both samples have a volume of 5.0 liters. Which statement concerning these samples is not true?
a- each weighs the same amount
b- each has the same # of atoms of gas
c- the density of the neon is greater than that of the helium
d- each sample contains the same # of moles of gas
e- none of the above
Well, I thought that the weight is the number under the symbol on the periodic table, which would be 4.003 for helium and 20.18 for neon. And that would make a not true. But I also thought that the number above the symbol on the periodic table is the number of atoms in the element, which would be 2 for helium and 10 for neon. So b would not be true. But I cant have two answers so I have to be wrong somewhere in this thinking...
a is indeed wrong. The number above the atom symbol is the atomic number, the number of protons in the nucleus.
B is true by Avagradros hypothesis
e is wrong also.
Good observation! Let's break down the statements one by one to determine the correct answer.
a) "Each weighs the same amount": You correctly pointed out that the weight of an element is represented by the atomic mass on the periodic table. The atomic mass of helium (He) is indeed 4.003 amu, while that of neon (Ne) is 20.18 amu. Therefore, statement a is not true since the two samples have different weights.
b) "Each has the same number of atoms of gas": As you also mentioned, the number above the symbol on the periodic table represents the number of atoms in one mole of the substance, known as Avogadro's number. The atomic number of helium is 2, indicating that it consists of 2 atoms per mole, while neon has an atomic number of 10, meaning there are 10 atoms per mole. Therefore, statement b is not true since the two samples have a different number of atoms.
c) "The density of neon is greater than that of helium": Density is defined as mass divided by volume. As we determined earlier, neon has a higher atomic mass than helium. However, both samples are at the same temperature, pressure, and volume. Since the volume is the same, mass is the determining factor for density. As helium has a lower mass than neon, its density will be lower. Therefore, statement c is not true.
d) "Each sample contains the same number of moles of gas": The number of moles of a substance can be calculated using the ideal gas law equation, which states that PV = nRT, where P is pressure, V is volume, n is the number of moles, R is the ideal gas constant, and T is temperature. Based on the given information, both samples have the same temperature, pressure, and volume. Therefore, the number of moles of gas in each sample is the same. So, statement d is true.
From the analysis above, we can conclude that the statement "each weighs the same amount" (a) is not true. Therefore, the correct answer to the question is a: each weighs the same amount.