Question

A liter of methane gas, CH₄, at STP contains more atoms of hydrogen than does a liter of pure hydrogen gas, H₂, at STP. Using Avogadro’s law as a starting point, explain why.

a) The molar mass of methane is higher than that of hydrogen.
b) Methane has more moles of atoms than hydrogen.
c) Hydrogen molecules dissociate into atoms at STP.
d) Methane undergoes a chemical reaction with hydrogen.

Answer 1

A liter of methane gas, CH₄, at STP contains more atoms of hydrogen than a liter of pure hydrogen gas, H₂, at STP because methane contains four hydrogen atoms per molecule while hydrogen gas contains only two hydrogen atoms per molecule.

Step-by-step explanation:

According to Avogadro's law, equal volumes of gases at the same temperature and pressure contain the same number of particles (atoms or molecules). Therefore, a liter of methane gas, CH₄, at STP contains the same number of particles as a liter of pure hydrogen gas, H₂.

However, since methane contains four hydrogen atoms per molecule while hydrogen gas contains two hydrogen atoms per molecule, a liter of methane gas contains more atoms of hydrogen than a liter of pure hydrogen gas.

Answer choices a) and b) are not accurate explanations. The molar mass of a substance is irrelevant in this case since Avogadro's law states that equal volumes of gases have equal numbers of particles regardless of mass. Methane does have more moles of atoms than hydrogen since it contains four atoms per molecule, while hydrogen gas contains only two atoms per molecule.

Answer choices c) and d) are also not accurate explanations. Hydrogen molecules do not dissociate into atoms at STP, meaning that a liter of hydrogen gas contains two hydrogen molecules, each composed of two atoms. Methane does not undergo a chemical reaction with hydrogen under normal conditions.