Determine the gram formula mass of a compound that has a density of 0.650 g/ L at STP?

Answer 1

Assuming this is a gas, we can apply the ideal gas law to figure this out:

PV = nRT Where P is pressure (1 bar), V is the volume of the gas, R is the gas constant (0.083 L-bar/mol-K), and T is the temperature in K.

If n = number of moles, and the number of moles is mass(m)/Molar Mass(MM), then:

PV = nRT
PV = mRT/MM
and:
(P x MM)/RT = m/V
if mass is divided by volume (like the above), we get density (d) in grams/liter.

So,
P x MM/RT = d
MM = dRT/P

So that's how we can figure out the molar mass of the substance.. all we have to do now is plug in the numbers.

MM = dRT/P

d = 0.650 g/L
R = 0.083 L-bar/mol-k
P = 1 bar (standard pressure)
T = 273.15K (standard temperature)
so,

MM = (0.650) x (0.083) x (273.15)/ 1 bar
MM = 14.74 g/mole

Answer 2

Since 1 mole occupies 22.4 L at STP, we multiply 0.65 g x 22.4 to get the nominal g-mole wt. It is about 15.