A sample of gas has a mass of 0.565 g . Its volume is 119 mL at a temperature of 86 ∘C and a pressure of 749 mmHg?

Mass, m = 0.565 g
Volume, V = 119 mL = 0.119 L
Absolute temperature, T = (273 + 86) K = 359 K
Pressure, P = 749/760 mmHg
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
Hence, M = mRT/(PV)

Molar mass, M = 0.565 × 0.08206 × 359 / [(749/760) × 0.119] g/mol = 142 g/mol

To do this problem you would use the ideal gas law which is PV=nRT. Where,
P= 749mmHg (The pressure that was given in the question)
V= 0.119L (Must be in Litres so 119mL was converted into Litres)
n= ? (I am assuming this is your unknown that you are trying to find)
R= 62.4 (This is just a constant)
T= 359K (This must be in kelvin so 86∘C was converted into kelvins)

PV=nRT

Rearrange the formula to make n the subject:
n=PV/RT

Enter your numbers:
n=(749mmHg*0.119L)/(62.4*359K)
n=0.00398 mols