I need help. I’m very confused with these questions

Question

I’m very confused on this question: 

1.83cm3 of a liquid of density of 0.803gcm-3 was injected into a syringe maintained at 200 celcius. The liquid vaporised completely and occupied 808.54cm3 at 1.20atm. a) calculate the volume that this sample of vapour will occupy at stp and hence calculate the molar mass of the liquid.

Answer 1

Assuming ideal gas behavior, you can use PV = nRT and solve for n (moles) or you can use P1V1/T1 = P2V2/T2

P = 1.20 atm

V = 808.54 cm3 = 0.80854 L

T = 200 C = 473 K

R = 0.0821 Latm/Kmole

Solving for n = PV/RT = (1.20 atm)(0.80854L)/(0.0821 Latm/Kmole)(473 K)

n = 0.02498 moles of gas

Using the other approach (which is what is suggested in the problem) and substituting P2 = 1 atm and T2 = 273 K (for STP) and solve for V2, you have...

(1.20atm)(0.80854L)/(473K) = (1atm)(V2)/(273) and solving for V2 you get 0.5599 L 

And converting this to moles at STP you have 0.5599 L x 1 mole/22.4 L = 0.02499 moles (SAME AS BEFORE)

 

What is the mass was added initially?  1.83 cm3 x 0.803 g/cm3 = 1.469 g added

1.469 g/0.02498 moles = 58.8 g/mole = molar mass