Let's </span>assume that the given gas is an ideal gas. Then we can use ideal gas equation,<span> PV = nRT<span> </span> Where,
P = Pressure of the gas (Pa)
V = volume of the gas (m³)
n = number of moles (mol)
R = Universal gas constant (8.314 J mol</span>⁻¹ K⁻¹)<span>
T = temperature in Kelvin (K) <span> The given data for the gas </span></span>is,<span>
P = 777 torr = 103591 Pa
V = </span>125 mL = 125 x 10⁻⁶ m³<span>
T = (</span>126 + 273<span>) = 399 K
R = 8.314 J mol</span>⁻¹ K⁻¹<span>
n = ?
By applying the formula, 103591 Pa x </span>125 x 10⁻⁶ m³ = n x 8.314 J mol⁻¹ K⁻¹ x 399 K<span> n = 3.90 x 10</span>⁻³<span> mol
</span>Moles (mol) = mass (g) /
molar mass (g/mol)<span>
Mass of the gas = </span><span>0.281 g </span>Moles of the gas = 3.90 x 10⁻³ mol <span>Hence, molar mass of the gas = mass / moles = 0.281 g / </span>3.90 x 10⁻³ mol <span> = 72.05 g/mol
Refer to a modern periodic table for the relative atomic mass of magnesium () and chlorine ():
: .
: .
In other words, the mass of of atoms would be (approximately) .
Likewise, the mass of of atoms would be approximately .
One formula unit of the ionic compound includes exactly as many atoms as there are in the given formula. The formula mass of a compound is the mass of of the formula units of this compound.
The formula includes one atom and two atoms.
Hence, every formula unit of would include the same number of atoms: one atom and two atoms. There would be of atoms and of atoms in of formula units.
Thus, the mass of of formula units would be equal to the mass of of atoms plus the mass of of atoms. (The mass of of each atom could be found from the relative atomic mass of each element.)
.
In other words, the formula mass of is .
Therefore, the number of formula units in of would be:
.
Multiple by Avogadro's Number to estimate the number of formula units in :