Question

determine the molar value of a gas if a 0.25 gram sample occupies 0.2 litres at STP​

Answer 1

First calculate the moles of the gas using the gas law, PV=nRT, where n is the moles and R is the gas constant. Then divide the given mass by the number of moles to get molar mass.
Calculate Moles
Given/Known
mass:0.950 g
pressure:P=0.985 atm
volume:V=232mL×1L1000mL=0.232 L
gas constant:R=0.082057338 L atm K−1mol−1
temperature:T=95oC+273.15=368 K
Unknown
n
Equation
PV=nRT
Solution
Rearrange the equation to isolate n and solve.
n=PVRT
n=0.985atm×0.232L0.082057338L atmK−1mol−1×368K=0.007568 mol
(I'm keeping a couple of guard digits to reduce rounding errors. I'll round to three significant figures at the end.)