Question

An experiment shows that a 250 ?ml gas sample has a mass of 0.430 g at a pressure of 736 mmhg and a temperature of 28 ?c.

Answer 1

What we're looking for here is the gas sample's molar mass given its mass, pressure, volume, and temperature. Recalling the gas law, we have

$PV = nRT$ or
$n = \frac{PV}{RT}$

where R is 0.08206 L atm / mol K, P is the given pressure, T is the temperature, and V is the volume.

Before applying the values given, it is important to make sure that they are to be converted to have consistent units with that of R. 

Thus, we have

P = 736/ 729 = 0.968 atm
T = 28 + 273.15 = 301.15 K
V = 250/1000 = 0.250 L

Now, applying these converted values into the gas law, we have

$n = \frac{(0.968 atm)(0.250 L)}{(0.08206 L.atm/mol.K)(301.15 K)}$
$n = 0.00979 moles$

Given that the mass of the sample is 0.430 g, we have

$molar mass = \frac{mass}{number of moles}$ 
$molar mass = \frac{0.430}{0.00979} = 43.9$

Thus, the gas sample has a molar mass of 43.9 g/mol.