A 0.435 g sample of a gas occupies 55.0 ml at standard temperature and 45.6 mmhg. Determine the molar mass of the gas.?

Consider the gas sample :
Mass, m = 0.435 g
Volume, V = 55.0 ml = 0.0550 L
Temperature, T = 273.15 K
Pressure, P = 45.6 mmHg = 45.6/760 atm = 0.0600 atm
Gas constant, R = 0.08206 L atm / (mol K)
Molar mass, M = ? g/mol

PV = nRT and n = m/M
Then, PV = (m/M)RT
Molar mass, M = mRT/(PV) = 0.435 × 0.08206 × 273.15 / (0.0600 × 0.0550) g/mol = 2950 g/mol

(Please check the question as the answer is unreasonable great. Substance with such a great molar mass should not be a gas.)

Molar mass.....

When you use R, simply use the value of R which matches the pressure units in your question. For pressure in torr (aka mm Hg), R = 62.36 Ltorr/molK. The following are well work remembering.
R = 8.314 LkPa/molK
R = 0.08206 Latm/molK
R = 62.36 Ltorr/molK

PV = nRT ........... ideal gas equation
PV = mRT / M .... n=m/M ... m=mass, M=molar mass
M = mRT / (PV)... solve for M
M = 0.435g x 62.36 Ltorr/molK x 273.15K / 45.6 torr / 0.0550L
M = 2950 g/mol .... the answer is rounded to three significant digits

This seems high for the molar mass of most common substances which might be gases at 0C.

At standard temperature and pressure, the volume of one mole of a gas is 22.4 liters. Let’s convert the volume of the gas from milliliters to liters.

V = 0.055 L

Standard pressure is 760 mm Hg. Let’s use the following equation to determine the volume of this gas at standard pressure.

P1 * V1 = P2 * V2
45.6 * 0.055= 760 * V2
V2 = 2.508 ÷ 760 = 0.0033

To determine the number of moles, divide this number by 22.4.

n = 0.0033 ÷ 22.4

This is approximately 1.47 * 10^-4 mole. To determine the mass of one mole, divide 0.435 grams by the number of moles.

Molar mass = 0.435 ÷ (0.0033 ÷ 22.4)

This is approximately 2952 grams. This is too high for a gas. 45.6 mmhg is a very very low pressure. Something is wrong with this problem!

Ideal gas law
PV = nRT
n = number of moles
R = gas constant = 0.08206 (atm∙L)/(mol∙K)
T = temperature in kelvins
P = absolute pressure in atm
V = volume in liters

45.6 mmHg = 0.06 ATM
STP = 0ºC and 1 ATM

n = PV/RT = (0.06)(0.055) / (0.08206)(273.16) = 0.0001472 mol